Add a String trait to avoid duplicating ~str, @str, &static/str operations

[catamorphism]

I'm going to naively suggest this because it seems like a better idea than making a copy of the whole str library for things that return @str. Now people can tell me why that's a bad idea :-)

[bstrie]

Can you be more specific about what you have in mind? Is this just an empty String trait to be used as a typeclass for generic string functions, or is there more to it?

[bstrie]

This should probably wait until the DST proposal #6308 is implemented.

[huonw]

Most functions in std::str are either methods on &str or functions taking &str, and there is now a Str trait that provides .as_slice() to do an explicit coercion of a generic string to &str (if it is ever necessary), so this can probably be closed.