feat: Make get_next/get_prev mor generic such that borrowed forms of keys can be used

[mlgiraud]

Title is self explanatory. If possible, it would be nice to have a small bugfix release for this. The change should be fully backwards compatible.

[coveralls]

coverage: 84.176% (+0.1%) from 84.074%
when pulling 7411260 on mlgiraud:feat/borrowed_keys
into 2398556 on jneem:main.

[jneem]

Thanks for the PR! I'm not sure about it being fully backwards-compatible, because if someone calls set.get_prev(x.into()) then type inference could fail with the new signature. (cargo semver-checks doesn't complain, but it also doesn't complain if I change the argument type to usize, so I think it's just incomplete in this case)

That said, I don't have any objection to bumping the version number.

[jneem]

It took me a while, sorry, but this is now in 3.0.0