[draft] Make Standard Library Index Types Hashable

Hello!

I’d like to propose adding Hashable conformance to the standard library’s index types, making subscripts in key path expressions more useful with the standard library’s collections. Note that this will not affect the requirements for Collection’s associated Index type, only concrete types in the stdlib.

Proposal draft is here: https://github.com/natecook1000/swift-evolution/blob/756370431682f41cfd9a63e39b81a96ccae467d9/proposals/0000-stdlib-index-types-hashable.md
Implementation here: https://github.com/apple/swift/compare/master...natecook1000:nc-index-hashable

Thanks!
Nate