Current implementation of IK export is a waste of performance

[Jason0214]

Current support for IK (#63) is implemented by calling blender API bake_action to baking constraint to actions, however, blender bake_action inner calls frame_set() for every frame. So call bake_action for every object has constraints is a waste of performance,

there are two possible way to solve it:

1. for blender > 2.79.1, there exist API bake_action_objects to bake multiple objects at once
2. simple use the approach of BetterCollada addon, we iteratively call frame_set() to export all the animation, give up the current implementation of evaluating fcurves

[Jason0214]

excited to see godot have native support for IK (godotengine/godot#18699), if we can export IK without baking then everything would be perfect