Deprecated/moved functions in collections::str

Breaking changes:
* Deprecated `str::from_utf8_owned` in favor of `String::from_utf8`
* Deprecated `str::from_chars` in favor of `String::from_chars`
* Deprecated `str::from_char` in favor of `String::from_char` and `.to_string()`
* Deprecated `str::from_byte` in favor of `String::from_byte`

Questions:
* Should we also move `from_utf16` and `from_utf16_lossy` to `String`?
* And `from_utf8_lossy`? I am not convinced of this, because it doesn't return a String (it returns a MaybeOwned)

[breaking-change]