2022-08-08

Auto merge of #12956 - oxalica:feat/la-arena-apis, r=lnicola

More methods and traits for `la_arena::ArenaMap`

Continue of #12931. Seems that I forgot some methods in the previous PR :(

I also changed `ArenaMap::insert` to return the old value, to match the map-like collection API of std. **So this is a breaking change.**

r? `@lnicola`