[RFC] add IndexAssign trait

[japaric]

Do not merge until its RFC has been approved

Introduce an unstable IndexAssign trait that lets users overload the a[b] = c expression.

Restore IndexMut implementations on HashMap/BTreeMap and also make them implement the IndexAssign trait, all these operations are now supported:

// insertion
map[key] = value
// apply mutating method to value associated to `key`
map[&key].foo_mut()// get mutable reference to value associated to `key`&mut map[&key]

---

RFC: rust-lang/rfcs#1129

cc @sfackler@gankro@nikomatsakis@aturon

[rust-highfive]

r? @alexcrichton

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[japaric]

RFC at rust-lang/rfcs#1129

[alexcrichton]

/me gasps

Nice!

r? @nikomatsakis, @nrc, and/or @pnkfelix

[alexcrichton]

cc @aturon

[Gankra]

🌞

[bors]

☔ The latest upstream changes (presumably #25609) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #26411) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #26575) made this pull request unmergeable. Please resolve the merge conflicts.

[Gankra]

(not closing this one since it looks like its RFC is getting into FCP soon)

[bors]

☔ The latest upstream changes (presumably #28602) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

I'm going to close this for now while we sit on the RFC, but this should always be here for posterity at least!