type safe Machine IL, code generator, and runtime

[Stephen Weeks]

Here's a list that I'd had sitting around for a while of things that
are not statically checked in MLton's backend/codegen/runtime, but it
would be nice if they were.

handlers
calling conventions
	argument/result passing
	continuation and handler used or not
limit checks
gc interface
gc itself
signal handling
bogus values needed for useless analysis
array initialization
bounds checks
efficient (data)type representations

I'm not going anywhere with this, I just thought it was an interesting
list.