To reduce the size of an rb_node, drop the color field. Set the least
significant bit in the pointer to the node from its parent to indicate
that the node is red. Have the tree rotation macros leave the
old-parent/new-child node red and the new-parent/old-child node black.

This change makes RB_LEFT and RB_RIGHT no longer assignable, and
RB_COLOR no longer defined. Any code that modifies the tree or
examines a node color would have to be modified after this change.

Reviewed by: markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D25105