Correctly infer lifetimes, & disambiguate result type in try-from.

[mrenow]

Thanks for the crate! New to rust and repo maintenance, but hoping that the following changes can be helpful.

Noticed that that the following example infers the lifetime of <'b> for Command, but doesnt pick up the lifetime of 'a

#[subenum(Command)]
pub enum Expression<'a, 'b> {
    #[subenum(Command)]
    CommandWithNoArgs(&'b SomeCommand<'a>) // 'a will not be registered, won't compile.
    ...
}

I have handled the respective case and written some tests.

I also noticed that a local definition of Result was clashing with the try-from signature, so I disambiguated the type.