EnumString should derive From<&str> instead of TryFrom<&str> when #[strum(default)] is present

[MingweiSamuel]

If you derive EnumString on a enum with a #[strum(default)] it generates a TryFrom<&str> implementation which can never return an error. It would make sense to generate From<&str> instead

This would be a breaking change

[k9withabone]

I think strum should still generate a FromStr and TryFrom<&str> implementation, but with the error type as Infallible, in addition to generating a From<&str> (and maybe From<String>).

This would allow me to use strum in a library I'm writing without having strum types as part of the API.