IntInf asserts

[Matthew Fluet]

> Looks good to me.  This cleans up the current world and makes
> integration with they new approach to IntInf easy.

Could also help with threads.  As best I can make out, creating a new
thread or copying the current thread enters the runtime and then invokes
ensureFree (which does a limit check and possible GC).  With a primitive
for querying the current thread's size, we could make thread creation act
like the new approach to IntInfs; i.e., when the primitive is invoked,
sufficient heap space is guaranteed and the primitive returns the new heap
pointer.  I don't know if it would buy us much besides a potentially
cleaner interface; the primitives would no longer need to be considered
entersRuntime, and a straight C call is slightly cheaper than a runtime
call.