Replace `bitmaps::Bitmap` with a simplified bitmap implementation for
the 256 wide use case needed by boreal.

This does a few things:
* Provide a simplified interface for Bitmap based on what's used by
  boreal
* Replaces a bunch of casts & slice indexing that could cause future
  issues with compile-time validation (by moving from indexing into the
  Bitmap by usize to u8)
* Removes a MPL2 licensed dependency (which can be incompatible with