Make Clock a generic parameter

[blind-oracle]

This makes a Clock trait a generic parameter, allowing to use e.g. FakeRelativeClock in tests to avoid waiting.
Public API gets default_with_clock() method that allows creating a builder with a custom Clock.

Clock defaults to DefaultClock as before, so API shouldn't break, but since most of public types got additional generic parameter - some code that references them might break, so it's a breaking change.

I'm not sure if it's possible to avoid that since Clock generic parameter boils down to RateLimiter from governor crate and RateLimiter doesn't expose any specific trait it seems so we can't just Box<dyn ...> it... maybe I am missing something here.

Fixes #50

[benwis]

Apologies for the delay in responses. If you could get the conflicts resolved, I'll get this merged in.