Prelude: rename and consolidate extension traits

This commit renames a number of extension traits for slices and string slices, now that they have been refactored for DST. In many cases, multiple extension traits could now be consolidated. Further consolidation will be possible with generalized where clauses.

The renamings are consistent with the [new `-Prelude` suffix](https://github.com/rust-lang/rfcs/pull/344). There are probably a few more candidates for being renamed this way, but that is left for API stabilization of the relevant modules.

Because this renames traits, it is a:

[breaking-change]

However, I do not expect any code that currently uses the standard library to actually break.
Would this be a good time to consolidate `Str` and `StrPrelude`? (or can we not do that?)