Delegate str:Index(Mut) to SliceIndex<str>
Delegate str:Index(Mut) to SliceIndex<str>

Move any extra logic that the former had into the latter, so they're consistent.

GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust