eager_macro_rules!: Selective implementations

[Emoun]

Allow the user to choose whether each rule should be implemented both normally and eagerly, just normally, or just eagerly:

eager_macro_rules!{$eager_1
    macro_rules! some_macro {
        {   // The default behaviour should be both
            ...
        } => { ... };
       @not_eager {   // Does not support eager
            ...
        } => { ... };
        @only_eager{   // Only supports eager, does not support non-eager
            ...
        } => { ... };
    }
}