Binary: Use ByteString's copy in getBS
Binary: Use ByteString's copy in getBS

It's unclear how much of an effect on runtime this will have, but if
nothing else the code generation may be a tad better since the system's
`memcpy` will be used.

Test Plan: Validate

Reviewers: simonmar, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2401

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