FastString: Supply mconcat implementation
FastString: Supply mconcat implementation

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2389

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