Add SampleTest and update macro

[AnIrishDuck]

Most of this PR is the SampleTest implementation, which was basically copied with some fixes from the quickcheck tester. There are several notable changes:

• Just using the Debug impl on tuples means there's no need to define a separate shrink function for each Testable. This simplifies the implementations and associated macro.
• We switch to an iterative shrinking procedure instead of a recursive one. This kills two birds with one stone:
  • Shrinking procedures that accidentally return infinite iterators will still eventually halt.
  • Stack overflows are no longer possible (see Stack overflow in quickcheck case shrinking BurntSushi/quickcheck#285 for an example of this in quickcheck)

Aside from that, the #[sample_test] macro gets much simpler as we no longer need to derive Arbitrary newtypes for each sampler. This also makes it more ergonomic: you can now define values directly and use function calls instead of the prior lazy_static requirement.

The only downside is when using once_cell::sync::Lazy or lazy_static, you need to call .clone() (as we expect the sampler by-value and these constructs return a Deref). I don't think this is a major problem as the function-call or direct-value use cases are probably the common path.

[jblondin]

looks interesting! I look forward to playing around with it

[jblondin]

ooh, I've never played with std::panic stuff, fun!

[AnIrishDuck]

Haha yeah. This was stolen from quickcheck. Useful for when a test run panics (e.g. an assert fails); we don't want to bubble the panic up, we want to treat it as a failure during the shrinking procedure.