Rts: Consistently use StgWord for sizes of bitmaps

A long debate is in issue #8742, but the main motivation is that this
allows for applying a patch to reuse the function scavenge_small_bitmap
without changing the .o-file output.

Similarly, I changed the types in rts/sm/Compact.c, so I can create
a STATIC_INLINE function for the redundant code block:

        while (size > 0) {
            if ((bitmap & 1) == 0) {
                thread((StgClosure **)p);
            }
            p++;
            bitmap = bitmap >> 1;
            size--;
        }
Rts: Reuse scavenge_small_bitmap (#8742)

The function was inlined at two places already. And the function is
having the STATIC_INLINE annotation, so the assembly output should.
be the same.

To convince myself, I did diff the output of the object files before
and after the patch and they matched on my 64-bit Ubuntu 13.10 machine,
running gcc 4.8.1-10ubuntu9.

Also, I had to move scavenge_small_bitmap up a bit since it's not in any
.h-file.

While I was at it, I also applied the analogous patch for Compact.c.
Though I had to write `thread_small_bitmap` instead of just moving it.
First NON-WORKING attempt of primitive for pushStackFrame