No functional changes. This PR replaces the uses of the C-style `BLI_BITMAP` with `blender::BitVector` in `keyframing.cc`. Note that in `BKE_animsys_nla_remap_keyframe_values` I had to add code that maps from one to the other because I don't feel comfortable ripping out the `BLI_BITMAP` from `NlaEvalChannelSnapshot->remap_domain` Pull Request: https://projects.blender.org/blender/blender/pulls/118957