Implement `Index` for `SliceIndex` types for `str`

[alercah]

While writing a generic implementation today, I was surprised that you can't do s[i] for i: SliceIndex<S>, if s is of slice type &S. Doing s.get(i).unwrap() is an easy enough workaround, but feels really bad. Replacing the various explicit impls with one for SliceIndex would feel much nicer.

[eddyb]

cc @rust-lang/libs

[sfackler]

The bounds on [T]'s get and Index impls seem the same?

https://doc.rust-lang.org/std/primitive.slice.html#method.get
https://doc.rust-lang.org/std/primitive.slice.html#impl-Index%3CI%3E

[SimonSapin]

Perhaps @alercah meant for str rather than slices? https://doc.rust-lang.org/std/primitive.str.html#impl-Index%3CRange%3Cusize%3E%3E

[alercah]

Ah, yes, I did. I mistakenly assumed they are in fact the same. :)

[sfackler]

Ah yeah, that seems like an oversight!