rb_tree: reduce duplication in balancing code

Change RB_INSERT_COLOR and RB_REMOVE_COLOR so that the blocks of code
that are identical except for left and right being exchanged are made
only one block with a variable to indicate left- or right-handedness.

Rename RB macros so that those not intended for external use begin
with an underscore.

Add comments to the balancing code so that another might understand it.

Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D36393