For T78995 we want to change the data structure of keylists to
improve performance. (Probably a Vector with bin-search capabilities).
This patch hides the internal structure of the keylists behind `AnimKeylist`
structure. This allows us to change the internals without 'breaking' where it is
being used.
The change adds functions to create, free, find and walk over the
keylist.
Reviewed By: sybren
Maniphest Tasks: T78995
Differential Revision: https://developer.blender.org/D11974
