This adds `BitSpan` and `MutableBitSpan`. They work essentially the same as
the normal `Span` and `MutableSpan`, but work on individual bits instead
(the smallest type `Span` can handle is one byte large).
This also splits up `BLI_bit_vector.hh` and introduces two new headers:
`BLI_bit_ref.hh` and `BLI_bit_span.hh`.
The goal here is to make working with dynamically sized bit masks more
convenient. I'm mainly working on this because I might want to use this
in #104629. It can also be used to cleanup function signatures that
currently take a reference to a `BitVector`. Like with `Span` vs. `Vector`,
it is better to pass a `BitSpan` to function than a `const BitVector &`.
Unit tests for the new code are included.
Pull Request #104671
891f47b801