Standardize newick string serialization

[recursion-ninja]

There is currently a Newick serialization type-class in pcg-serialize.

We should require that newick serialization place the larger subtree on the right. Additionally, if two subtrees are of equal size, the "lesser" tree should be placed on the left according to the lexicographic ordering of it's leaf labels.

This the convention in phylogenetics. The added benefit is that serialized Newick trees will have a canonical representation, allowing for equality checking of Newick strings.

Some though can be given to expanding this serialization standarization to eNeiwck strings.