Normalize output

[make-github-pseudonymous-again]

Currently some big rectangles are output as a stream of corner touching rectangles when the middle snake is empty.
There is already a simplify function available in test/src/_fixtures.js that merges this stream into the maximally larger one. This should perhaps be made available in the API. We could also combine this with a transpose functionality in case we implement #26. Merging rectangles is not a necessity so I do not think it should be implemented in the middle of the Hirschberg dc.