v0.15.2

This patch adds failure blobs to the native backend. A failure blob is a
base64 string encoding the counterexample. With the new print_blob setting
enabled, a failing #[hegel::test] prints it as a copy-pasteable attribute:

To reproduce this failure, add the attribute below #[hegel::test]:
    #[hegel::reproduce_failure("AAEC…")]

#[hegel::test]
#[hegel::reproduce_failure("AAEC…")]
fn my_test(tc: hegel::TestCase) {
    let x: i32 = tc.draw(hegel::generators::integers());
    assert!(x < 100);
}

The attribute can be stacked to keep track of several failures; only the
first one replays — delete them one by one as you fix the failures. A blob
that decodes but no longer reproduces a failure is reported as a failing run.

Over the C ABI, hegel_failure_reproduction_blob reads the blob off a failure
and hegel_test_case_from_blob replays it.

We only guarantee the compatibility of the failure blob within a specific Hegel
version.