feat: add serialization (save/load)

[okdistribute]

This adds serialization to micromerge.

let binary = doc.save()
let copy = Micromerge.load(binary)

I had to remove the Symbol definitions because they were not serializing correctly. An alternative approach we can take, if you really do need Symbols, is to use symbol.description: https://sequoia.makes.software/two-painful-ways-to-misuse-javascripts-symbol-descriptions/, but that would have taken more work so I punted on it. Tests pass without using Symbol anyway.