Add newick format

Ready for review

I still need to add `read` and `write` as well as deprecate `to_newick` and `from_newick`

@wasade I am particularly interested in any comments you might have.

fixes #627 
They started very different and converged over time. Based on that I don't know if I want to explicitly couple them lest they need to diverge again. If I pulled it out as `_newick_operators` behaviour may change if the maintainer is unaware that it is used twice.