widen key constraint to 'any' for a standard map

[laher]

This adjustment in response to #25

I went ahead and changed the key constraint to any for a non-sorted Map, and it turned out to be much less work than I expected, particularly because you can bring your own Hasher.

• In the interest of my sanity I also moved the sorted map into a separate file.
• I don't know how well this works, I just worked on it until it complied and tests passed.
• I only had to change one func body here - func ( h *defaultHasher[K]).Equal(a, b K). Seems fine I think

[laher]

superceded by #29