swift234

Throw the switch to start using generic slices everywhere.  Out with the old,
in with the new!

Random notes:
1. I had to XFAIL 4 IRGen tests; I'll file radars on them.
2. I preserved SliceStringByte and renamed it StringByteData because
   it has some String-specific functionality.
3. There's a small IRGen patch included which fixes a couple of runtime
   failures involving generic array new.



Swift SVN r2644