KeyValueMap easier to populate/transform to

Thank you for `Tags::parse(..)`.

We probably don't need/want any out-of-the box parsing for key-value pairs, since the separators... may vary. But how about
- `KeyValueMap::new(given_map: HashMap)` that would take its ownership (especially since the current implementation is a wrapper), and optionally:
- a conversion function like `KeyValueMap::new(it: Iterator) -> Self`? (Or implementing `TryFrom`, but something tells me that `TryFrom` is not a good way to go here.)

Start by reading the current KeyValueMap implementation and the Tags::parse(..) API, paying attention to how the wrapper stores its data. Compare the proposed HashMap ownership and iterator conversion options with existing callers. Done means the supported construction paths are decided, implemented, and covered by tests for the requested key-value inputs.

Written by the indexing model from the issue text.