Add more methods matching the standard HashMap

[cuviper]

• pub fn get_key_value<Q: ?Sized>(&self, key: &Q) -> Option<(&K, &V)>

  • Stabilized in Rust 1.40.
  • This is a middle-ground between get and our own get_full that
    includes the index. It's easy enough to add for compatibility.

• pub fn remove_entry<Q: ?Sized>(&mut self, key: &Q) -> Option<(K, V)>

  • Stabilized in Rust 1.27.
  • For IndexMap, this also adds swap_/shift_remove_entry choices.
  • These are also a middle-ground between (swap_/shift_)remove and
    our own swap/shift_remove_full that include the index. We don't
    have a plain remove_full though.

[cuviper]

I'm going ahead with the merge, because there's no real API design here. This is just matching std methods, plus swap/shift semantics, and the implementations are trivial.