FastString: Reduce allocations of concatFS
FastString: Reduce allocations of concatFS

Instead of unpacking and then repacking we simply concatenate all of the
individual ByteStrings.

snoyberg/ghc,ezyang/ghc,shlevy/ghc,sgillespie/ghc,snoyberg/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,vTurbine/ghc,sdiehl/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,vTurbine/ghc,vTurbine/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,snoyberg/ghc,snoyberg/ghc,mettekou/ghc,shlevy/ghc,ezyang/ghc,sgillespie/ghc,sgillespie/ghc,sgillespie/ghc,vTurbine/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,olsner/ghc,mettekou/ghc,vTurbine/ghc,sgillespie/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sgillespie/ghc,vTurbine/ghc,sdiehl/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,olsner/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc,mettekou/ghc,sdiehl/ghc,sgillespie/ghc,snoyberg/ghc,ezyang/ghc