This adds a new `blender::BitVector` data structure that was requested
a couple of times. It also replaces usages of `BLI_bitmap` in C++ code.
See the comment in `BLI_bit_vector.hh` for more details about the
advantages and disadvantages of using a bit-vector and how the new
data structure compares to `std::vector<bool>` and `BLI_bitmap`.
Differential Revision: https://developer.blender.org/D14006
