pseudo-register addressing

[Matthew Fluet]

> It wouldn't be trivial, but we could set it up so that we have all of the
> "real" stack value at the bottom of the stack and then all the pseudo-reg
> stack values at the top.  By the time we get to a limit-check or a
> non-tail call, all the pseudo-regs will be dead, so we can over-write the
> top of the stack with function arguments, etc.  

At the same time we still need to know the largest size the entire stack
frame can get for the appropriate stack check.