Remove OwnedStr trait

This trait was only implemented by `String`. It provided the methods
`into_bytes` and `append`, both of which **are already implemented as normal
methods** of `String` (not as trait methods). This change improves the
consistency of strings.

This shouldn't break any code, except if somebody has implemented
`OwnedStr` for a user-defined type.
Deprecated `str::raw::from_c_str`

Replaced by `String::from_c_str`

[breaking-change]
Deprecated `str::raw::from_utf8_owned`

Replaced by `String::from_bytes`

[breaking-change]
Deprecated `str::raw::from_byte`

Use `String::from_bytes` instead

[breaking-change]