Return Cow<str> from to_case method.

[chernoivanenkoofficial]

If a referenced str is already in requested case, is there any reason why it shouldn't be returned in borrowed view?

[rutrum]

The current algorithm for case transformation doesn't perform any overhead to check any properties of the input string. Even if the operations don't make any changes, there are still some borrowing and cloning that happens are part of the conversion. There's improvements to be made in this regard, but in general I don't think the overhead of checking if it's already a case is worth the savings of not copying the string. I suspect most use cases of case conversion you explicitly know that a string isn't the right case, which is why you convert it. Is there a use case you are thinking of where this performance advantage would be substantial?