🐄 Convert Backend's &str to Cow instead

[Jasper-Bekkers]

Fixes #44

This is a breaking change to the Backend trait, but it stems from the realization that the i18n macro expansion always would call Cow::from on the result of both available_locales as well as translate.

This PR just moves that responsibility over to the Backend trait directly, this unlocks exactly the "reloadable" backend that the original issue was talking about.

Before

pub trait Backend: Send + Sync + 'static {
    /// Return the available locales
    fn available_locales(&self) -> Vec<&str>;
    /// Get the translation for the given locale and key
    fn translate(&self, locale: &str, key: &str) -> Option<&str>;
}

After

pub trait Backend: Send + Sync + 'static {
    /// Return the available locales
    fn available_locales(&self) -> Vec<Cow<'_, str>>;
    /// Get the translation for the given locale and key
    fn translate(&self, locale: &str, key: &str) -> Option<Cow<'_, str>>;
}