StrSlice should implement ImmutableSlice

[nwin]

Although these traits share a lot of common functionality they are totally separated such that additional functionality that could be shared between them has to be implemented twice.

An example is the “eager”-slicing as discussed on reddit. It would be convenient both for &str and &[T] (given it makes sense to slice strings anyway).

[steveklabnik]

Changes of this nature would require an RFC nowadays, please persue one if this is still relevant and you still feel strongly about it. Thanks!