GC tweaks

[Henry Cejtin]

I  am  putting  in  asserts  (for  the  alignment)  in the move code in gc.c.
Actually, I will probably just make it an inline function.

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

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.  The
only case seems to be if stack->reserved or  stack->used  where  not,  and  I
don't  see  how  that  could happen.  (I tested it in the self compile and it
never happened thare, but that isn't much of an argument.)