Add an #[explicit_test_case] feature.

[DRMacIver]

Depends on #143. Review that one first.

This mirrors Hypothesis's example decorator. It lets you explicitly
provide a test case as a set of inputs to your test function.

Here's it in action. Consider the following:

const BAD_VALUE: u32 = 124352450;

#[hegel::test]
#[hegel::explicit_test_case(
    x = BAD_VALUE
)]
fn my_test(tc: hegel::TestCase){
    let x = tc.draw(hegel::generators::integers::<u32>());
    assert!(x != BAD_VALUE);
}

If you run this you will see:

let x = BAD_VALUE; // = 124352450

thread 'my_test' (15608116) panicked at examples/explicit_test_case.rs:9:5:
assertion failed: x != BAD_VALUE

It will not work in all cases. Currently in some of those cases it will likely produce very bad error messages. I've tried to do my best to think of the common cases for this and provide good error messages there, ideally at compile time, but there are 100% going to be some missing ones that we can improve over time.