Implement IndexMut for String and str.

Implement IndexMut for String and str.

... matching the existing Index impls.
There is no reason not to if String implement DerefMut.

The code removed in `src/librustc/middle/effect.rs` was added in #9750
to prevent things like `s[0] = 0x80` where `s: String`,
but I belive became unnecessary when the Index(Mut) traits were introduced.