Enhancement/Feature: port upstream IndexMap methods such as shift_remove

[prutschman]

In #321 it was mentioned that there were upstream methods from IndexMap that hadn't been ported yet, shift_remove in that particular case. I don't know if this is important enough to be considered for 1.0, but I thought I'd re-surface the issue for discussion.

retain being in-order does mean there are workarounds; I'm currently simulating shift_remove_index(0) with a combination of .first and .retain(|key| key != first_key)