Backport python 3.7 dataclass

A new `dataclass` decorator was recently accepted into python 3.7 (https://www.python.org/dev/peps/pep-0557/).

The implementation seems similar to coconut's implementation of the `data` type (subclassing the namedtuple) but they differ in terms of their syntax. I was just wondering whether `coconut` would keep the current syntax (for backward compatibility) or would it be changing to the new syntax (to avoid feature duplication). I just asked this question because I'm curious.