rb_tree: optimize tree rotation

[dougm]

Summary

The RB_ROTATE macros begin with fetching a field via a pointer. In almost most cases, that value is one that is already in a register, and the compiler cannot infer it. So, to eliminate those needless fetches, have the caller of the RB_ROTATE macros present the data in the third macro parameter, rather than having the macro fetch it.

And a few minor style cleanups.

[dougm]

Rename variables in REMOVE_COLOR (elm->child, sib->elm) to make it look more like the INSERT_COLOR code.

[dougm]

Purify the patch.

[dougm]

Find a better (smaller) way to set 'child' in RB_INSERT_COLOR. Reduce lip3 size from 8830 to 8734.