migrate internals to immutable types

[pgte]

A CRDT type is defined in a stateless manner. All return values are supposed to be immutable, and there's a lot of cloning happening just to make sure we don't mutate past state.
Instead of that, I think we could have better performance (and less error-prone) if we used immutable types internally.

[robchristian]

Looks like immutable is already in use. Strongly suggest the use of immer instead. Immutability is important, but immutable.js was a train wreck.
https://immerjs.github.io/immer/docs/introduction