Immutable components

[mikera]

We should remove mutable state from components.

Mutable state was originally included in components for performance reasons. However this seems to be counter-productive:

• The performance gain is probably quite small as long as mutable vectors are used to accumulate gradient values (which is the expensive mutable step)
• It means that defensive copying of components is required for concurrent use.
• It's generally unidiomatic in Clojure to use mutable objects

This will require some significant refactoring, but is probably worth it to improve the overall Nurokit API