The runtime deals with 1-bit pointer bitmaps and 2-bit heap bitmaps
that have entries for both pointers and mark bits.
Each byte in a 1-bit pointer bitmap looks like pppppppp (all pointer bits).
Each byte in a 2-bit heap bitmap looks like mpmpmpmp (mark, pointer, ...).
This means that when converting from 1-bit to 2-bit, as we do
during malloc, we have to pick up 4 bits in pppp form and use
shifts to create the mpmpmpmp form.
This CL changes the 2-bit heap bitmap form to mmmmpppp,
so that 4 bits picked up in 1-bit form can be used directly in
the low bits of the heap bitmap byte, without expansion.
This simplifies the code, and it also happens to be faster.