[Effects handling] Eliminate Context::getHandled().
[Effects handling] Eliminate Context::getHandled().

We don't want to expose the notion of "handles everything", because
that's a honeypot for messing up 'async' checking when making a context
change for error-handling, and vice versa. Instead, provide an API to
produce a derived context where all errors are handled, and use it for
(e.g.) try/try!/try? and exhaustive do-catches.

nathawes/swift,hooman/swift,hooman/swift,parkera/swift,CodaFi/swift,apple/swift,gregomni/swift,atrick/swift,gregomni/swift,glessard/swift,allevato/swift,allevato/swift,ahoppen/swift,benlangmuir/swift,glessard/swift,nathawes/swift,xwu/swift,roambotics/swift,rudkx/swift,allevato/swift,atrick/swift,roambotics/swift,atrick/swift,tkremenek/swift,benlangmuir/swift,rudkx/swift,CodaFi/swift,jmgc/swift,rudkx/swift,benlangmuir/swift,JGiola/swift,tkremenek/swift,roambotics/swift,ahoppen/swift,glessard/swift,apple/swift,jmgc/swift,roambotics/swift,gregomni/swift,nathawes/swift,xwu/swift,gregomni/swift,gregomni/swift,jmgc/swift,xwu/swift,CodaFi/swift,xwu/swift,xwu/swift,apple/swift,ahoppen/swift,tkremenek/swift,gregomni/swift,rudkx/swift,tkremenek/swift,roambotics/swift,nathawes/swift,JGiola/swift,allevato/swift,apple/swift,glessard/swift,parkera/swift,JGiola/swift,JGiola/swift,nathawes/swift,jmgc/swift,hooman/swift,parkera/swift,ahoppen/swift,tkremenek/swift,parkera/swift,apple/swift,xwu/swift,tkremenek/swift,glessard/swift,glessard/swift,roambotics/swift,CodaFi/swift,jmgc/swift,JGiola/swift,jmgc/swift,parkera/swift,parkera/swift,benlangmuir/swift,rudkx/swift,allevato/swift,parkera/swift,apple/swift,CodaFi/swift,jmgc/swift,ahoppen/swift,CodaFi/swift,allevato/swift,allevato/swift,atrick/swift,benlangmuir/swift,hooman/swift,rudkx/swift,parkera/swift,nathawes/swift,nathawes/swift,ahoppen/swift,hooman/swift,CodaFi/swift,tkremenek/swift,xwu/swift,hooman/swift,benlangmuir/swift,JGiola/swift,hooman/swift,atrick/swift,atrick/swift