Loosen constraint on the Key type in rb & avl tree

[destroycomputers]

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.

[laytan]

Note that the where clause will now allow pointer types through, which are not comparable with less/greater than. It is probably fine though, it will still error somewhere else.