More instance-gate fiddling.  This must be one of the most
tiremsome bits of the entire compiler, and I appear to be
incapable of modifying it without getting it wrong at least
once.

Still, this commit does tidy things up a bit.

* The type renamers (rnHsType, etc) have moved from RnSource
  into a new module RnTypes.

* This breaks a couple of loops, and lets us nuke RnSource.hi-boot.
  Hurrah!

Simon