Add fuzzing and proptesting of unsafe code #1

From the README:

This crate contains a decent amount of unsafe code from handling its internal linked list, and the unsafe code has diverged quite a lot from the original linked-hash-map implementation. It currently passes tests under miri and sanitizers, but it should probably still receive more review and testing, and check for test code coverage.

We want to:

• Review existing test patterns
• Introduce new property tests (using hegel or proptest) and fuzzing (with afl.rs or cargo-fuzz)
• Focus on exercising unsafe code