Replace tip/internal node metadata structures in empress.js with arrays?

[fedarko]

Since the keys in these Objects are the postorder position of a node, it should be feasible to just store these as Arrays (where the i-th entry holds data for the i-th node in the postorder position), analogous to @kwcantrell's comment at #339 (comment).

[fedarko]

...Actually not sure this is a good idea (at least in terms of saving space in the HTML output by the python code), because if the amount of feature metadata is sparse then there'll be a lot of undefineds scattered throughout the array. Would probably be easier to just stick with storing feature metadata as Objects, and IIRC these are represented the same? way as arrays internally in JS so it probably wouldn't make much of a difference for performance.