Give til::bitmap custom allocator support and add til::pmr::bitmap (#…

…8787)

`til::details::bitmap<Allocator>` will use `Allocator` for its
`dynamic_bitset`, and it will use a rebound allocator for its run storage.

Allocator should be an allocator type storing `unsigned long long`, the
backing store type for `dynamic_bitset`.

I've introduced a type alias, `til::bitmap`, which papers over the
allocator choice for all existing code. I've also introduced a second
type alias, `til::pmr::bitmap`, which lets a consumer use the C++
polymorphic allocator system.

I chatted with @miniksa about whether to keep the "full" allocator
version in `details` or not. We decided that for the simplicity of the
`til` namespace, we would. If anybody has a compelling reason to use
`til::details::bitmap<Allocator>` directly, we can re-evaluate this
decision.