Rewriting bitmap_allocator

[Benjamin De Kosnik]

> With increasing memory sizes, and increasing difference between the
> size of CPU caches and main memory, improving locality and reducing
> the per-object allocation overhead seems to have become an important
> idea. The current bitmap_allocator has been crashing on me (even
> though it provides excellent cache locality).

Maybe step one is to add a test that crashes on you when you
use it.

> With that in mind, I would like to propose a re-write of the
> bitmap_allocator<> now that I have a better idea of how it can be
> implemented. I would be happy to present a proposal for the same (with
> details) if it is a thing of interest.

Please. Bonus if you can figure out a way to remove __mini_vector or
scope it such that it is clearly and implementation detail.

-benjamin