This patch fixes Trac #909.  The problem was that when compiling 
the base package, the handling of wired-in things wasn't right;
in TcIface.tcWiredInTyCon it repeatedly loaded GHC.Base.hi into the
PIT, even though that was the very module it was compiling.

The main fix is by introducing TcIface.ifCheckWiredInThing.

But I did some minor refactoring as well.