Support Cows with arbitrary lifetimes

[hcsch]

Fixes #86.

Since the lifetime parameter of Cow is only for the borrowed variant, it is irrelevant for the impl. This also allows for Cow<'static, str> as can be seen when testing with the following code:

#[derive(Arbitrary)]
pub struct StaticCow {
    cow: Cow<'static, str>,
}

#[test]
fn static_cow() {
    // Last byte is used for length
    let raw: Vec<u8> = vec![97, 98, 99, 100, 3];
    let static_cow: StaticCow = arbitrary_from(&raw);
    assert_eq!(
        Cow::<'static, str>::Owned("abc".to_string()),
        static_cow.cow
    );

    let (lower, upper) = <StaticCow as Arbitrary>::size_hint(0);
    assert_eq!(lower, 8);
    assert_eq!(upper, None);
}

If desirable I can also add this test case to tests/derive.rs (which is also where arbitrary_from is taken from).

[nagisa]

This seems okay at a first glance but I wonder if we might want to actually implement Cow::Borrowed stuff later down the line. It seems like an average fuzzer may be have an easier time seeing through the Cow if it didn't also involve an allocation for a typical str.

[hcsch]

That is a good point you have there, I didn't really think about that. Thanks for bringing it up!

[hcsch]

I suppose an implementation for Cow<'static, T> would be in conflict with such a plan, at least as long as one can't specialize implementations based on lifetimes (edit: e.g. one impl for 'static, one for arbitrary 'a). Should I close this then @nagisa?