Remove unnecessary trait bounds.

[afranchuk]

These changes mainly remove unnecessary trait bounds and change Copy to Clone where making a copy is necessary.

Note that it is possible to remove the need for Clone entirely by changing the output type of certain functions to references rather than values, and changing the predicate functions to take a reference. However those would be breaking changes, so I left it at these compatible changes for now.

For kicks I did try making those other changes above, and the changes themselves are easy to make, but changing all the tests to compare correctly is a bit more involved. Also worth noting is that if you were to make those changes to return references, it would be very convenient to provide something like:

impl<'a, I, O: Clone> Parser<'a, I, &'a O> {
    pub fn cloned(self) -> Parser<'a, I, O> {
        self.map(|v| v.clone())
    }
}