impl Lex and Parse for references

Should Lex and Parse have the following blanket impl?:

```rust
impl<T, O> Parse for &T
where
    T: Parse<Output = O>,
{
    type Output = O;

    fn parse<'i>(&self, input: &'i str) -> ParseResult<'i, Self::Output> {
        (*self).parse(input)
    }
}
```

Should we have combinators that take references to parsers/lexers?