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