Remove `Clock: Clone` requirement

[max-heller]

Closes #231

Also adds RateLimiter::clock() to access the clock, enabling usages like the following:

let clock = FakeClock::new();
let limiter = RateLimiter::new_with_clock(quota, clock);
limiter.clock().advance(by);

This would, for instance, enable using a FakeRelativeClock(AtomicU64) instead of having to wrap the counter in an Arc as is currently done.