Reduce rbt macrology

There are lots of macros in rbt.c that are syntatic vinegar for structure accessors. They do not help with brevity nor with readability. In particular, pointer chasing reads better like left->right instead of RIGHT(left)

So, use Coccinelle to get rid of them.

There are some follow-on improvements to the comments around the macro definitions, and a style fix for a function that used to be a macro.