Bug: `get_prev` returns incorrect results

[bojanserafimov]

Judging by docstrings, for any m: OrdMap, m.get_prev(&key) should return the same results as m.range(..=key).next_back(). But that's not the case, and get_prev sometimes returns None where range returns Some.

I created a repo that reproduces the problem. Hopefully you find it helpful for debugging: https://github.com/bojanserafimov/im_crate_bug_report