GC tweaks

[Stephen Weeks]

> I don't like the test using to terminate the loop:
>     from >= limit
> It isn't strictly ANSII for one thing.  Can't I just copy size + skip  bytes?

That is fine, although it seems like quite a waste to copy the skip bytes since
they will never be looked at.  With lots of threads (stacks) this could cost.

> Wait,  it  seems that skip is zero in the case of arrays.  The only time skip
> is non-zero is for stacks.

Right, stacks have extra unused space for pushing more frames.

> You claimed that size is not always divisible by 4, but I never  saw  a  case
> where  it  wasn't  in the self compile, and I don't see how it could be.

It happens if a char or word8 is put on the stack.