[build-script-impl] Optimize setting -> variable name lookups.
[build-script-impl] Optimize setting -> variable name lookups.

 - The current behavior of mapping via `tr` was very slow when called over and
   over for each option. This matters for SR-237 where I would like
   `build-script` to take over the top-level build process control loop, which
   involves executing `build-script-impl` multiple times.

 - This patch optimizes the lookup in two ways:

   1. It does the setting to variable conversion in a single pass, for all
      options at once.

   2. It builds a cache of the setting to variable name conversion (as an
      exploded associate array), and uses that when doing variable assignment.

 - This patch speeds up `build-script-impl --dry-run` by 2x on one test case.

bitjammer/swift,devincoughlin/swift,arvedviehweger/swift,gregomni/swift,allevato/swift,brentdax/swift,tinysun212/swift-windows,gribozavr/swift,Jnosh/swift,deyton/swift,kperryua/swift,devincoughlin/swift,lorentey/swift,lorentey/swift,Jnosh/swift,xedin/swift,xedin/swift,frootloops/swift,codestergit/swift,kperryua/swift,deyton/swift,sschiau/swift,austinzheng/swift,hughbe/swift,sschiau/swift,bitjammer/swift,shajrawi/swift,alblue/swift,kstaring/swift,return/swift,roambotics/swift,stephentyrone/swift,kstaring/swift,codestergit/swift,lorentey/swift,huonw/swift,parkera/swift,tardieu/swift,parkera/swift,felix91gr/swift,shahmishal/swift,uasys/swift,tardieu/swift,huonw/swift,brentdax/swift,ben-ng/swift,jtbandes/swift,airspeedswift/swift,nathawes/swift,apple/swift,gmilos/swift,calebd/swift,jckarter/swift,therealbnut/swift,aschwaighofer/swift,gottesmm/swift,kperryua/swift,devincoughlin/swift,karwa/swift,IngmarStein/swift,modocache/swift,jckarter/swift,amraboelela/swift,hooman/swift,return/swift,ahoppen/swift,atrick/swift,shahmishal/swift,dreamsxin/swift,gribozavr/swift,bitjammer/swift,jmgc/swift,karwa/swift,xedin/swift,kstaring/swift,kstaring/swift,swiftix/swift,stephentyrone/swift,therealbnut/swift,huonw/swift,jtbandes/swift,alblue/swift,lorentey/swift,deyton/swift,therealbnut/swift,gribozavr/swift,allevato/swift,djwbrown/swift,gmilos/swift,alblue/swift,manavgabhawala/swift,tkremenek/swift,xwu/swift,parkera/swift,zisko/swift,jopamer/swift,ben-ng/swift,allevato/swift,kperryua/swift,tjw/swift,ahoppen/swift,russbishop/swift,gottesmm/swift,hughbe/swift,jopamer/swift,airspeedswift/swift,huonw/swift,shajrawi/swift,therealbnut/swift,sschiau/swift,tkremenek/swift,rudkx/swift,calebd/swift,return/swift,swiftix/swift,benlangmuir/swift,danielmartin/swift,swiftix/swift,manavgabhawala/swift,Jnosh/swift,nathawes/swift,CodaFi/swift,harlanhaskins/swift,manavgabhawala/swift,apple/swift,nathawes/swift,jmgc/swift,ben-ng/swift,milseman/swift,felix91gr/swift,practicalswift/swift,JaSpa/swift,austinzheng/swift,milseman/swift,practicalswift/swift,karwa/swift,aschwaighofer/swift,parkera/swift,return/swift,gottesmm/swift,swiftix/swift,zisko/swift,kstaring/swift,aschwaighofer/swift,Jnosh/swift,djwbrown/swift,arvedviehweger/swift,sschiau/swift,gregomni/swift,codestergit/swift,gregomni/swift,milseman/swift,sschiau/swift,zisko/swift,shahmishal/swift,lorentey/swift,kperryua/swift,frootloops/swift,jmgc/swift,sschiau/swift,roambotics/swift,tardieu/swift,alblue/swift,tkremenek/swift,devincoughlin/swift,jckarter/swift,arvedviehweger/swift,xwu/swift,practicalswift/swift,milseman/swift,bitjammer/swift,codestergit/swift,manavgabhawala/swift,arvedviehweger/swift,gottesmm/swift,djwbrown/swift,russbishop/swift,codestergit/swift,tinysun212/swift-windows,nathawes/swift,tkremenek/swift,rudkx/swift,hooman/swift,gottesmm/swift,Jnosh/swift,tjw/swift,jckarter/swift,ben-ng/swift,modocache/swift,rudkx/swift,jmgc/swift,alblue/swift,atrick/swift,arvedviehweger/swift,kperryua/swift,shajrawi/swift,shahmishal/swift,danielmartin/swift,gmilos/swift,gmilos/swift,CodaFi/swift,nathawes/swift,parkera/swift,kstaring/swift,tjw/swift,return/swift,alblue/swift,glessard/swift,xwu/swift,allevato/swift,austinzheng/swift,harlanhaskins/swift,Jnosh/swift,OscarSwanros/swift,swiftix/swift,xedin/swift,russbishop/swift,shajrawi/swift,airspeedswift/swift,huonw/swift,allevato/swift,tjw/swift,bitjammer/swift,gribozavr/swift,allevato/swift,rudkx/swift,hooman/swift,karwa/swift,natecook1000/swift,gmilos/swift,austinzheng/swift,brentdax/swift,amraboelela/swift,austinzheng/swift,natecook1000/swift,zisko/swift,return/swift,aschwaighofer/swift,milseman/swift,OscarSwanros/swift,IngmarStein/swift,JaSpa/swift,huonw/swift,hooman/swift,natecook1000/swift,modocache/swift,shahmishal/swift,roambotics/swift,JaSpa/swift,amraboelela/swift,JaSpa/swift,OscarSwanros/swift,jtbandes/swift,stephentyrone/swift,return/swift,glessard/swift,hooman/swift,stephentyrone/swift,hughbe/swift,djwbrown/swift,uasys/swift,ahoppen/swift,lorentey/swift,practicalswift/swift,alblue/swift,amraboelela/swift,bitjammer/swift,shajrawi/swift,aschwaighofer/swift,tinysun212/swift-windows,JGiola/swift,jtbandes/swift,xedin/swift,modocache/swift,brentdax/swift,gmilos/swift,uasys/swift,gribozavr/swift,brentdax/swift,natecook1000/swift,modocache/swift,ben-ng/swift,devincoughlin/swift,JGiola/swift,airspeedswift/swift,manavgabhawala/swift,jtbandes/swift,stephentyrone/swift,devincoughlin/swift,amraboelela/swift,felix91gr/swift,tjw/swift,amraboelela/swift,codestergit/swift,milseman/swift,kperryua/swift,benlangmuir/swift,hughbe/swift,shahmishal/swift,dreamsxin/swift,gregomni/swift,jtbandes/swift,OscarSwanros/swift,allevato/swift,manavgabhawala/swift,uasys/swift,IngmarStein/swift,frootloops/swift,apple/swift,felix91gr/swift,airspeedswift/swift,deyton/swift,OscarSwanros/swift,xwu/swift,shajrawi/swift,jopamer/swift,jopamer/swift,atrick/swift,calebd/swift,JGiola/swift,frootloops/swift,karwa/swift,benlangmuir/swift,xedin/swift,apple/swift,aschwaighofer/swift,jopamer/swift,JGiola/swift,harlanhaskins/swift,ben-ng/swift,austinzheng/swift,rudkx/swift,felix91gr/swift,danielmartin/swift,tkremenek/swift,nathawes/swift,calebd/swift,hughbe/swift,tardieu/swift,deyton/swift,ahoppen/swift,tardieu/swift,JGiola/swift,natecook1000/swift,harlanhaskins/swift,karwa/swift,djwbrown/swift,practicalswift/swift,glessard/swift,practicalswift/swift,uasys/swift,IngmarStein/swift,modocache/swift,jckarter/swift,xwu/swift,modocache/swift,zisko/swift,danielmartin/swift,jckarter/swift,sschiau/swift,harlanhaskins/swift,xedin/swift,airspeedswift/swift,lorentey/swift,natecook1000/swift,benlangmuir/swift,shajrawi/swift,tardieu/swift,tardieu/swift,djwbrown/swift,tinysun212/swift-windows,apple/swift,stephentyrone/swift,therealbnut/swift,huonw/swift,russbishop/swift,lorentey/swift,atrick/swift,practicalswift/swift,kstaring/swift,jmgc/swift,milseman/swift,russbishop/swift,CodaFi/swift,glessard/swift,OscarSwanros/swift,IngmarStein/swift,shajrawi/swift,arvedviehweger/swift,CodaFi/swift,shahmishal/swift,tinysun212/swift-windows,russbishop/swift,benlangmuir/swift,gribozavr/swift,harlanhaskins/swift,roambotics/swift,tkremenek/swift,gottesmm/swift,felix91gr/swift,devincoughlin/swift,arvedviehweger/swift,JaSpa/swift,ahoppen/swift,aschwaighofer/swift,ben-ng/swift,therealbnut/swift,felix91gr/swift,brentdax/swift,swiftix/swift,jckarter/swift,tinysun212/swift-windows,zisko/swift,CodaFi/swift,calebd/swift,amraboelela/swift,CodaFi/swift,rudkx/swift,xwu/swift,glessard/swift,therealbnut/swift,gmilos/swift,apple/swift,djwbrown/swift,manavgabhawala/swift,glessard/swift,russbishop/swift,bitjammer/swift,hooman/swift,practicalswift/swift,hughbe/swift,JaSpa/swift,roambotics/swift,danielmartin/swift,deyton/swift,jopamer/swift,IngmarStein/swift,benlangmuir/swift,xwu/swift,sschiau/swift,gribozavr/swift,gregomni/swift,CodaFi/swift,OscarSwanros/swift,JGiola/swift,devincoughlin/swift,natecook1000/swift,tinysun212/swift-windows,zisko/swift,hooman/swift,gottesmm/swift,parkera/swift,nathawes/swift,roambotics/swift,shahmishal/swift,swiftix/swift,calebd/swift,tjw/swift,karwa/swift,harlanhaskins/swift,jopamer/swift,ahoppen/swift,jmgc/swift,parkera/swift,Jnosh/swift,tkremenek/swift,codestergit/swift,JaSpa/swift,stephentyrone/swift,deyton/swift,uasys/swift,hughbe/swift,frootloops/swift,frootloops/swift,calebd/swift,austinzheng/swift,atrick/swift,jtbandes/swift,IngmarStein/swift,tjw/swift,jmgc/swift,airspeedswift/swift,danielmartin/swift,gribozavr/swift,atrick/swift,uasys/swift,brentdax/swift,danielmartin/swift,parkera/swift,gregomni/swift,frootloops/swift,xedin/swift,karwa/swift