str: Add method .into_owned(self) -> ~str to Str

The method .into_owned() is meant to be used as an optimization when you
need to get a ~str from a Str, but don't want to unnecessarily copy it
if it's already a ~str.

This is meant to ease functions that look like

  fn foo<S: Str>(strs: &[S])

Previously they could work with the strings as slices using .as_slice(),
but producing ~str required copying the string, even if the vector
turned out be a &[~str] already.

I don't have any concrete uses for this yet, since the one conversion I've done to `&[S]` so far (see PR #8203) didn't actually need owned strings. But having this here may make using `Str` more attractive.

It also may be worth adding an `into_managed()` function, but that one is less obviously useful than `into_owned()`.