StrBuf::from_owned_str makes a copy

[SimonSapin]

… despite its documentation claiming it doesn’t.

Ideally, we should:

• Add a no-copy Vec::from_owned_slice, based on Vec::from_raw_parts
• Change StrBuf::from_owned_str to use Vec::from_owned_slice

Alternatively (or in the meantime), remove the claim from the documentation.

[alexcrichton]

The comment is referring to a post-DST world in which it doesn't make a copy. In the interim (before it's implemented), it will be forced to make a copy.

Closing because this will hopefully be implemented soon!