While investigating something else I found that 'typeSize' was
allocating like crazy.  Stupid becuase it should allocate precisely
nothing!!

Turned out that it was because typeSize and coercionSize were mutually
recursive across module boundaries, and so could not benefit from the
CPR property.  To fix this I moved them both into TyCoRep.

It's not critical (because typeSize is really only used in
debug mode, but I tripped over and example (T5642) in which
typeSize was one of the biggest single allocators in all of GHC.
And it's easy to fix, so I did.