Structural:
- Fix unconditionally enabled MSRV 1.85 incompatible code.

Feature changes:
- Add `{Bump,BumpSlice}::capacity` for synchronized allocators.
- Add `{Bump,BumpSlice}::data_ptr` for synchronized allocators.
- Add `{Bump,unsync::Bump}::from_maybe_uninit` to initialize an allocator
  in-place of an existing reserved space for it.
- Add `{Bump,BumpSlice}::from_memory` to initialize an allocator in-place of a
  potentially fitting memory location.