The current constraint on the `init_ordered' function accepts only
numeric types. While one still can init a non-numerically-keyed tree by
passing a comparator explicitly, the reason to disallow non-numeric
ordered types is nowhere to be found.
Add a red-black tree with configurable $Key and $Value.
Also includes tests that verify it maintains RB invariants, doesn't leak.
Originally based on the CC0 implementation from literateprograms.org.
But reworked to the same API used by @Yawning's excellent `core:container/avl` for ease of use.
