Use an MD5 hash instead of Object.hash for model hashes

[jakemac53]

We can easily swap out the actual hash algorithm here now, in one place.

This is a bit slower (unsurprisingly) than using Object.hash etc, a few hundred milliseconds in total per iteration, but is a fairer representation of what we actually want.

[jakemac53]

There is one failing test here - false and null generate the same digest. What value do you think we should use for the bytes of null/bool?

[jakemac53]

We could include the type as a part of the digest for everything.... or maybe booleans get an extra byte, or we use const [1] and const [2] as for true/false when it comes to hashing, instead of 0 and 1.

Edit: I went with the [1] and [2] solution for false/true