Expose &str with 'static lifetimes where possible

[MOZGIII]

Using this crate is a relatively high-perf setting where doing a clone of a string to pass it around it kind of unwanted. It would be great if the crate would allow to just use static references for things that allow it.

For instance: country.un_locale() could easily return &'static str instead of &str.

[pouriya]

Hmm. yeah. I think we need to change this section. Could you do that?

[MOZGIII]

I use only a small part of the API surface - so I don't really know where to look for all the instances where this would be relevant. So, I'd avoid taking this task.

[MOZGIII]

I mean, I suspect there are also other placrs where this would matter, besides the file you linked

[pouriya]

Done