StrAllocating::to_string and into_string should be removed

[SimonSapin]

… since they are redundant with StrAllocating::to_owned and StrAllocating::into_owned. The latter names are more specific, assuming we talk about String as an "owned string" and &str as a "string slice".

[alexcrichton]

The to_owned method has been deprecated and is slated to removal, and it appears that the into_owned method was accidentally not marked #[deprecated] just yet. I believe the current plan is to remove the owned functions and leave the string functions. That is not set in stone though.

[SimonSapin]

I see, thanks. If it’s already #[deprecated], there is no need to keep this bug open. I blame #14493.