Fix bug in remove_index for OrdMap

[dsarlis]

Fixes #124

When removing a value and rebalancing the tree, a value could be moved to a node that doesn't have the required free capacity, therefore resulting in a panic.

[dsarlis]

This is fixing the repro mentioned in the issue and there's only a single test failing for im-rc when I ran the tests locally. I believe this patch is in the right direction, so any help to make sure all tests pass is welcome.

[bodil]

I've got no tests failing after this fix, including after I added the repro case for #124, so I'll take that as a positive sign. 😊

[dsarlis]

Thanks @bodil! Yes, I'd noticed that the tests hadn't failed on CI either, but I remember there was a failure locally for me. Maybe I did something wrong though. If all tests pass this sounds like a good sign to me too.

[bodil]

CI is finding new bugs now, though, but I can't say whether they were caused by this commit or would have been there with the old code too - most likely the latter. They seem extremely nondeterministic, in that I'm not able to reproduce them on my local machine from the dataset the CI proptest is reporting - as the tests are using a std::collections::HashMap as a control, it might very well be down to the exact ordering of inputs as determined by the randomised seed the hasher is using... oh joy. I've redone the tests, and hopefully the fuzzer finds a reproducible case soon.

Anyway, this definitely fixed one of the bugs, so good work. 😊