Truly Immutable Cursors

[jameshopkins]

Fixes https://github.com/redbadger/immutable-cursor/issues/5

• [x] Remove the reference to the root cursor in each derived cursor, thus, disconnecting the point of cursor-level root atom mutations.
• [x] Integrate a Clojure inspired atom to manage mutation of the state tree, enabling cursors to be truly immutable.
• [x] Retain the concept of update serialisation within mutative methods, allowing for multiple synchronised updates in a single render cycle.

Immutability within cursors introduces more of a dependency on render cycles to create new derived cursors with updated sub-atoms.

TODO

• [x] Replace js-atom with an implementation whose interface allows for a) external persistence, b) managing serialisation of nested updates.

[jameshopkins]

@leebyron Would be interested to get your thoughts on the approach I've taken, when you have some time.

[leebyron]

I didn't give it a super close read, but from the overall approach this looks quite solid. I much prefer this style of cursored-atom to what Immutable.js is doing today.