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.
Similar to `Str` above: I hope to replace both with a more general conversion trait soon, but we're not quite ready to take that step.