Make sure all reducers are immutable

[effa]

Do we need immutability? If so, what is the best approach to make reducers working with lazy state immutable? Currently we are using ChainMap, which has several drawbacks. Should we consider a 3rd party library for immutable types (and namedtuples)? Or should we allow for impurere reducers? (Generally it's a question of immutable vs. Pythonic...)