- Change INT to a type that remembers whether it was boxed or not.
- We still need a way of specifying a plain unboxed int, so add a
"phony" UNBOXED_INT.  UNBOXED_INT is not usable as a variable type,
but can be used to specify argument and return types.  It's up to
the receiver (irgen or type analysis) to convert UNBOXED_INT to
a usable INT.  Right now that's only guarded with a number of
asserts (and not through, say, the C++ type system).