Want a homogeneous (bitmap-only) heap index

[stephenrkell]

Some heaps, like gcc's RTL heap, always allocate objects of the same (super)type so only a bitmap is really needed. This 'easy win' was already mooted in 2015's Onward! paper but I seem not to have got around to it yet.

(It would also be good to dig into the performance of generic-small, since I always felt it 'should have' been faster than it seemed to be in practice. An N-way comparison of indexers would probably be easier to achieve now... even for an ordinary malloc heap we could do all three approaches and compare.)