Blender Git Loki

Python API: implement `PoseBone.children` via `Bone.children`.

Currently `PoseBone.children` is implemented by a linear scan of
the list of armature bones. This is doubly inefficient, since
not only is it scanning all bones, the `obj.data.bones` list
is actually synthetic and generated from Bone children lists.

Instead, use the `Bone.children` native RNA property.

Differential Revision: https://developer.blender.org/D12727