TreeMap: find_equiv(_mut) for str-like keys

- it allows to lookup using any str-equiv object, making TreeMaps finally usable (for example, it is much easier to work with JSON with lookup values being static strs)
- actually provides pretty flexible solution which could be extended to other equivalent types (although it might be not that performant)
Let's please remove these `K: Str` implementations and leave it to another PR.