Consider adding a lifetime parameter to Arbitrary

[fitzgen]

If we tweaked the Arbitrary definition into this:

pub trait Arbitrary<'a>: Sized {
    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self>;

    // ...
}

Then we would allow implementing Arbitrary for things that borrow from the input string:

struct HasBorrow<'a> {
    borrow: &'a [u8],
}

impl<'a> Arbitrary<'a> for HasBorrow<'a> {
    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<HasBorrow<'a>> {
        let size = u.arbitrary_byte_size()?;
        let borrow = u.get_bytes(size)?;
        Ok(HasBorrow { borrow })
    }
}

There are two concerns, off the top of my head:

1. This is a breaking change, and shouldn't be done lightly. Perhaps delayed until we have more breaking changes that we can roll up into one release.

2. Right now, Arbitrary requires that types that implement it be 'static and I can't remember why. There sure are a bunch of errors when I remove this bound though. Anyways, this would also have to be removed.

[frewsxcv]

Done in #63