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Integrate hair collisions with cloth collision

This integrates hair collisions with the new cloth collision system,
greatly improving reliability, and reducing the amount of hair-specific
code paths in the cloth code.

The removes all the point constraint based collision stuff, instead
implementing segment impulse based collisions, using the same collision
response code as the normal cloth solver.

The hair system can now also collide with the emitter if it is a
collision object.

Reviewed By: mano-wii, Sebastian Parborg

Differential Revision: https://developer.blender.org/D6545