Clean up frozen-instance code #35

See dataclasses, new in Python 3.7. (This pretty much fills the same role as MacroPy's case classes.)

We could raise dataclasses.FrozenInstanceError where appropriate.

Also, e.g. unpythonic.llist.cons doesn't need the internal readonly flag; it's simpler to just call object.__setattr__ to init the read-only fields, so the custom __setattr__ can be simplified. (Should check also other datatypes that use this pattern, maybe something in unpythonic.collections?)

Also, must intercept also __delattr__ to properly emulate immutability. Not sure if we currently do this (probably not - hence the bug label, we need to check).