The `BKE_animsys.h` and `anim_sys.c` files already had a an "AnimData API" section. The code in that section has now been split off, and placed into `BKE_anim_data.h` and `anim_data.c`. All files that used to include `BKE_animsys.h` have been adjusted to only include the animation headers they need (sometimes none). No functional changes.