Macro combinedfun::parser_hint

macro_rules! parser_hint {
    (<Input = $I:ty, Error = $E:ty>) => { ... };
    (<Error = $E:ty, Input = $I:ty>) => { ... };
    (<Input = $I:ty>) => { ... };
    (<Error = $E:ty>) => { ... };
}

Allows helping Rusts type inference by returning an ε parser with the given type parameters. This is intended to be used with the overloaded operators.