explicit arrays and limit checks

[Stephen Weeks]

> But, in the explict branch, this is a serious problem, because the MACHINE
> IL is outputing runtime transfers with Prim.gcCollect with the arguments
> we need; but x86-mlton is assuming that any GC_collect is nullary and just
> filling in amount = 0, force = TRUE.  What it should really do is use
> arguments if they are there, otherwise use the default.

Makes sense, except that I don't see the overloading.  I changed the
basis library so that GC_collect is of type word * bool -> unit.  So,
all calls that the codegen sees should have two args.