Remove ImmutableJS

[hellosmithy]

Should we remove Immutable.JS in favour of either:

• vanilla JS
• an alternative library that's more compatible with RamdaJS etc. such as Seamless Immutable (https://github.com/rtfeldman/seamless-immutable)
• some other solution?

[hellosmithy]

Benefits of seamless-immutable seem to be:

• Maintains backwards compatibility with RamdaJS
• Just plain Arrays and POJOs with Object.freeze in development

Some possible cons or gotchas to be aware of:

• RamdaJS will still sometimes return a new non-immutable Array or Object
• It only supports Objects and Arrays. No Map, Set etc yet.
• Doesn't support objects with circular references (don't see this being a problem)