Feature request: try_insert method

[mb64]

The standard library hashmap and btree map recently got a nightly-only try_insert operation (here's the tracking issue), which makes it very convenient to insert something into a map, panicking in the case that it's already there:

the_map.try_insert(key, value).unwrap();

It would be nice to also have this for IndexMap.

[cuviper]

We're happy to match such methods once they're stable, but until then it's subject to change, and we don't want indexmap to get stuck with differences.