lib: improve the RB implementation

lib: improve the RB implementation

Switch the RB tree implementation completely to the new dlg@'s version
that uses pre-declared functions instead of macros for tree functions.

Original e-mail/diff:
https://marc.info/?l=openbsd-tech&m=147087487111068&w=2

Pros:

* Reduces the amount of code that the usage of those macros generate
* Allows the compiler to do a better compile-time check job
* Might have better i-cache utilization since the tree code is shared

Con:

* dlg@ benchmarks shows it has 'very slightly slower' insertions
* imported RB_* code must adapt the following calls:
  RB_INIT(), RB_GENERATE(), RB_ROOT(), RB_EMPTY(), make compare
  functions use 'const' (if not already) and maybe others.