Implement Clone for possible matchers

[kupiakos]

To simply reuse the same long matcher expression across multiple checks.

[bjacotg]

What about injecting an & before the matcher in verify_that!, assert_that! and expect_that!, like it is done for actual?

This would solve the problem for:

let actual = 2;
let matcher = eq(2);
assert_that!(actual, matcher);
assert_that!(actual, matcher); // both pass by ref so ok!

It wouldn't solve the issue for:

let matcher = eq(2);
assert_that!(vec![2, 2], [matcher.clone(), matcher]);

which will require implementing Clone for matchers.

The first one is fairly simple to implement right now. The second is no more complicated, but I will need to make all matcher function return concrete types as part of #323 fix, so I may save this for later.

WDYT?

[bjacotg]

I run into temporary lifetime issue by taking the matcher by reference. However, in #356, I just made all &impl Matcher implement Matcher which seems to fix both issues.

[kupiakos]

@bjacotg Implementing Clone might still be useful for other reasons, but matching on &Matcher works just as well for me!