minion_rs: preparing for property testing and fuzzing

[niklasdewally]

Property testing and fuzzing would both be sensible approaches to checking the type safety and parsing code of minion_rs, as previously discussed in #122.

The aim of this issue is to setup the boilerplate needed to use property tests in minion_rs in the future, and for it to be easily run through a fuzzer for exotic test case generation.

The following crates are useful here:

• https://rust-fuzz.github.io/book/cargo-fuzz/structure-aware-fuzzing.html
• https://proptest-rs.github.io/proptest/intro.html

Both crates contain their own "Arbitrary" traits that are used to generate test cases. However, the same implementation of Arbitrary can be used for both crates using https://github.com/graydon/proptest-arbitrary-interop.