GC tweaks

[Stephen Weeks]

> > If  you are really correct that the stack size can fail to be divisible by 4,
> > then this is a disaster.  It would mean that if you later push an int on  the
> > stack,  it  will not be aligned.  The Intel architecture will allow this, but
> > it will slow you down by a huge huge amount.
> 
> Well, this same sort of thing came up in the discussion about floats on
> the stack.  Objects on the stack are aligned relative to the stackTop, but
> frames aren't aligned.

Yeah, but they're not even aligned mod 4, whereas objects are at least aligned
mod 4.  I think it is easy to fix and am investigating.