Eliminate the color field from the RB element struct. Identify the
color of a node (or, really, the color of the link from the parent to
the node) by using one of the last two bits of the parent pointer in
that parent node. Adjust rebalancing methods to account for where
colors are stored, and the fact that null children have a color too.

Adjust RB_PARENT and RB_SET_PARENT to account for this change.

Reviewed by: markj
Tested by: pho, hselasky
Differential Revision: https://reviews.freebsd.org/D25418