generics: widen map key constraint to 'comparable'

[laher]

Addressing #25 (second attempt)

As suggested by @anacrolix after my previous attempt (#26):

I gave it a spin, it didn't work for the underlying user (although the stm code passed). I think the type can be widened for SortedMap (and any other variants too). Again, because comparers are manually given, the restriction on types belong to those implementations. I'm pretty sure it works for the builtin comparers too, as those use type sets to check for orderable types anyway. Short answer: I think K comparable can be used everywhere in immutable.

So I used comparable, but under the hood it only supports strings, the various integer types, and derived types.

As before, the defaultComparer panics if you try to use any other key type. You just need to write your own Comparer for other types.

[benbjohnson]

@laher Sorry for the late reply. AFAICT this looks ok. @anacrolix what do you think?

[anacrolix]

Give me a few days to try it out.

[BarrensZeppelin]

I've been using this for a while without issues. 👍

[benbjohnson]

Thanks @laher. I went ahead and merged. If anyone has any objections or changes, go ahead and open a new PR. 👍

[anacrolix]

This did solve the issue, I used it in anacrolix/stm@32174bf and anacrolix/dht@88ee338.

[laher]

Nice one. I just noticed that I didn't update the readme. I can PR it a bit later, and then maybe we need another release?

[benbjohnson]

I went ahead and cut a v0.4.1 release: https://github.com/benbjohnson/immutable/releases/tag/v0.4.1