MownStr: Maybe Owned String

This crate provides MownStr, a type for exchanging read-only strings that may be either borrowed or owned.

Contrarily to other types (such as for example Cow<str>), MownStr does not aim to be mutable nor generic, which allows it to be fast and lean.

Actually, a MownStr takes no more memory than a regular &str or Box<str>, and has a minimal runtime overhead. The drawback is that the maximum size of a MownStr is half the size of a regular str (which is still 8EiB on a 64-bit architectures...).