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)
This is no longer the case. The mutable version is almost identical now, except for a temporary variable to appease borrowck