Cleaned up str module a bit

Moved `StrSlice` doc comments from impl to trait.
Moved `OwnedStr` doc comments from impl to trait.
Normalized a few `StrSlice` method names from `FOO_reverse`, `FOO_rev` and `rFOO` to `FOO_rev`.

The doc comment changes make the source a bit harder to read, as documentation and implementation no longer live right next to each other. But this way they at least appear in the docs.