Allow indexing HashMap/BTreeMap mutably

[Ms2ger]

Note #23559. The suggestion is to use get_mut instead, but that's a lot uglier.

[Gankra]

tl;dr for anyone frustrated with this: blocked on IndexSet being fully defined to avoid Messes

[steveklabnik]

Triage: no change.

[dtolnay]

This is going to require an RFC. Let's track this as part of rust-lang/rfcs#997.

[BatmanAoD]

@dtolnay @Gankra It's not immediately clear to me from looking at RFC issue 997 why BTreeMap and HashMap can't yet implement the existing IndexMut trait; is there already discussion about the connection somewhere that you can point me to? If not, would one of you mind explaining the connection here?

[dtolnay]

IndexMut would make map[k] = v compile but panic on key not already present. Presumably a user would expect insertion into the map, like in C++ and Go and JavaScript and every other language with that syntax, not a panic. That is rust-lang/rfcs#997 and some designs are mutually exclusive with IndexMut.