Change Writeable::writeable_to_string to return a Cow

[sffc]

Also adds impl Writeable for str.

This enables writeable_to_string to be more efficient in certain cases.

Replaces part of #1438

[Manishearth]

I kinda feel like this is a method you're only going to call when you need something owned, but I guess this is fine.

[sffc]

I kinda feel like this is a method you're only going to call when you need something owned, but I guess this is fine.

The use case is in #1438. We need to look at the string the Writeable is going to append to the sink in order to modify the behavior in some way (such as switching between "y" and "e" in Spanish). It's nice to avoid the allocation.

[Manishearth]

Yeah I noticed, which is why I approved this, I just find it a little weird.