Always immutable (remove mutation special casing)

[chrisui]

I saw the note in the readme and just wanted to have an issue where desire for this can be tracked and an area for discussion. This would be my expectation from such a library - I would expect whatever I pass into these utilities to be immutably handled (whether an immutable.js data structure or other).

From the readme:

In the future we may opt for an entirely immutable approach for both forms of data, similar to the approach for lodash-fp

+1 from me