nonmoving: introduce a family of dense allocators

Supplement the existing power 2 sized nonmoving allocators with a family
of dense allocators up to a configurable threshold.

This should reduce waste from rounding up block sizes while keeping the
amount of allocator sizes manageable.

This patch:
  - Adds a new configuration option `--nonmoving-dense-allocator-count`
    to control the amount of these new dense allocators.
  - Adds some constants to `NonmovingAllocator` in order to keep
    marking fast with the new allocators.

Resolves #23340