Rolling back: Make FastString thread-safe.      This patch was the cause of the compile-time performance regression in #3796.  My guess is that it is due to the use of unsafePerformIO which traverses the stack up to the first update frame, and perhaps we have a deep stack when reading the dictionary from a .hi file.  In any case, since we're not relying on thread safety for FastStrings, I think the safest thing to do is back this out until we can investigate further.
Rolling back: Make FastString thread-safe.
    
This patch was the cause of the compile-time performance regression in
#3796.  My guess is that it is due to the use of unsafePerformIO which
traverses the stack up to the first update frame, and perhaps we have
a deep stack when reading the dictionary from a .hi file.  In any
case, since we're not relying on thread safety for FastStrings, I
think the safest thing to do is back this out until we can investigate
further.

wxwxwwxxx/ghc,gcampax/ghc,oldmanmike/ghc,sgillespie/ghc,forked-upstream-packages-for-ghcjs/ghc,shlevy/ghc,forked-upstream-packages-for-ghcjs/ghc,christiaanb/ghc,vikraman/ghc,acowley/ghc,gridaphobe/ghc,mettekou/ghc,AlexanderPankiv/ghc,holzensp/ghc,holzensp/ghc,vTurbine/ghc,AlexanderPankiv/ghc,AlexanderPankiv/ghc,tjakway/ghcjvm,TomMD/ghc,ml9951/ghc,spacekitteh/smcghc,sdiehl/ghc,mettekou/ghc,holzensp/ghc,lukexi/ghc,tjakway/ghcjvm,urbanslug/ghc,sdiehl/ghc,anton-dessiatov/ghc,jstolarek/ghc,vTurbine/ghc,spacekitteh/smcghc,GaloisInc/halvm-ghc,ezyang/ghc,bitemyapp/ghc,forked-upstream-packages-for-ghcjs/ghc,snoyberg/ghc,ilyasergey/GHC-XAppFix,gcampax/ghc,tjakway/ghcjvm,forked-upstream-packages-for-ghcjs/ghc,ghc-android/ghc,nkaretnikov/ghc,fmthoma/ghc,mcmaniac/ghc,shlevy/ghc,tjakway/ghcjvm,ekmett/ghc,mfine/ghc,urbanslug/ghc,wxwxwwxxx/ghc,shlevy/ghc,da-x/ghc,christiaanb/ghc,gridaphobe/ghc,ml9951/ghc,christiaanb/ghc,fmthoma/ghc,sdiehl/ghc,elieux/ghc,GaloisInc/halvm-ghc,nathyong/microghc-ghc,gcampax/ghc,siddhanathan/ghc,acowley/ghc,lukexi/ghc,nushio3/ghc,da-x/ghc,nathyong/microghc-ghc,ghc-android/ghc,gridaphobe/ghc,acowley/ghc,anton-dessiatov/ghc,nomeata/ghc,ilyasergey/GHC-XAppFix,lukexi/ghc,lukexi/ghc,gridaphobe/ghc,vikraman/ghc,frantisekfarka/ghc-dsi,nomeata/ghc,mfine/ghc,vikraman/ghc,da-x/ghc,bitemyapp/ghc,gridaphobe/ghc,frantisekfarka/ghc-dsi,mfine/ghc,fmthoma/ghc,ezyang/ghc,gridaphobe/ghc,nomeata/ghc,ml9951/ghc,ezyang/ghc,AlexanderPankiv/ghc,siddhanathan/ghc,mcschroeder/ghc,elieux/ghc,siddhanathan/ghc,snoyberg/ghc,frantisekfarka/ghc-dsi,snoyberg/ghc,urbanslug/ghc,wxwxwwxxx/ghc,ryantm/ghc,bitemyapp/ghc,nathyong/microghc-ghc,holzensp/ghc,da-x/ghc,ml9951/ghc,nathyong/microghc-ghc,hferreiro/replay,holzensp/ghc,hferreiro/replay,mfine/ghc,anton-dessiatov/ghc,ilyasergey/GHC-XAppFix,ekmett/ghc,TomMD/ghc,nkaretnikov/ghc,christiaanb/ghc,gridaphobe/ghc,da-x/ghc,wxwxwwxxx/ghc,elieux/ghc,green-haskell/ghc,gcampax/ghc,anton-dessiatov/ghc,lukexi/ghc,tibbe/ghc,acowley/ghc,AlexanderPankiv/ghc,mettekou/ghc,nushio3/ghc,sgillespie/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,nomeata/ghc,jstolarek/ghc,nushio3/ghc,snoyberg/ghc,gcampax/ghc,nushio3/ghc,vTurbine/ghc,wxwxwwxxx/ghc,olsner/ghc,shlevy/ghc,mcschroeder/ghc,ml9951/ghc,green-haskell/ghc,oldmanmike/ghc,mfine/ghc,nkaretnikov/ghc,nkaretnikov/ghc,snoyberg/ghc,spacekitteh/smcghc,anton-dessiatov/ghc,ml9951/ghc,mcmaniac/ghc,nomeata/ghc,mcschroeder/ghc,AlexanderPankiv/ghc,TomMD/ghc,ryantm/ghc,nushio3/ghc,urbanslug/ghc,sgillespie/ghc,jstolarek/ghc,olsner/ghc,sgillespie/ghc,nathyong/microghc-ghc,green-haskell/ghc,lukexi/ghc-7.8-arm64,ezyang/ghc,mettekou/ghc,fmthoma/ghc,sgillespie/ghc,oldmanmike/ghc,ilyasergey/GHC-XAppFix,ekmett/ghc,siddhanathan/ghc,elieux/ghc,tjakway/ghcjvm,snoyberg/ghc,forked-upstream-packages-for-ghcjs/ghc,sgillespie/ghc,ekmett/ghc,tibbe/ghc,ezyang/ghc,ghc-android/ghc,olsner/ghc,mcmaniac/ghc,ryantm/ghc,lukexi/ghc-7.8-arm64,mettekou/ghc,TomMD/ghc,elieux/ghc,lukexi/ghc-7.8-arm64,acowley/ghc,oldmanmike/ghc,fmthoma/ghc,ezyang/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,da-x/ghc,TomMD/ghc,hferreiro/replay,hferreiro/replay,oldmanmike/ghc,bitemyapp/ghc,spacekitteh/smcghc,mettekou/ghc,nushio3/ghc,frantisekfarka/ghc-dsi,nkaretnikov/ghc,urbanslug/ghc,shlevy/ghc,ekmett/ghc,ryantm/ghc,oldmanmike/ghc,hferreiro/replay,GaloisInc/halvm-ghc,siddhanathan/ghc,tibbe/ghc,shlevy/ghc,tibbe/ghc,nkaretnikov/ghc,olsner/ghc,olsner/ghc,hferreiro/replay,mfine/ghc,shlevy/ghc,wxwxwwxxx/ghc,hferreiro/replay,vikraman/ghc,green-haskell/ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,urbanslug/ghc,sdiehl/ghc,ghc-android/ghc,nkaretnikov/ghc,da-x/ghc,anton-dessiatov/ghc,elieux/ghc,vTurbine/ghc,fmthoma/ghc,sdiehl/ghc,acowley/ghc,mcschroeder/ghc,ml9951/ghc,fmthoma/ghc,gcampax/ghc,lukexi/ghc-7.8-arm64,nushio3/ghc,jstolarek/ghc,TomMD/ghc,mcschroeder/ghc,christiaanb/ghc,wxwxwwxxx/ghc,sdiehl/ghc,forked-upstream-packages-for-ghcjs/ghc,vTurbine/ghc,urbanslug/ghc,mfine/ghc,tjakway/ghcjvm,mettekou/ghc,frantisekfarka/ghc-dsi,mcmaniac/ghc,anton-dessiatov/ghc,GaloisInc/halvm-ghc,green-haskell/ghc,vTurbine/ghc,sdiehl/ghc,ml9951/ghc,elieux/ghc,nathyong/microghc-ghc,christiaanb/ghc,vTurbine/ghc,jstolarek/ghc,ghc-android/ghc,christiaanb/ghc,ryantm/ghc,vikraman/ghc,oldmanmike/ghc,forked-upstream-packages-for-ghcjs/ghc,gcampax/ghc,olsner/ghc,ghc-android/ghc,sgillespie/ghc,vikraman/ghc,tibbe/ghc,tjakway/ghcjvm,olsner/ghc,mcschroeder/ghc,nathyong/microghc-ghc,spacekitteh/smcghc,siddhanathan/ghc,mcmaniac/ghc,ezyang/ghc,vikraman/ghc,AlexanderPankiv/ghc,bitemyapp/ghc,acowley/ghc,lukexi/ghc-7.8-arm64,TomMD/ghc,ghc-android/ghc