* mst: more test coverage
In particular, ports the older fanout=32 corner-case tests to fanout=16.
I generated new paths with correct level and ordering, but haven't
actually verified that the resulting structures have the shape of the
ASCII diagrams in text.
* mst: additional comments, including TODO questions
* mst: default new MST layer to 'null' not 0
Looks like all current call sites of this 'create()' function override
the layer already. This is just making it explicit that the default,
null, means the layer has not been computed yet, distinct from being
layer=0.
* mst: remove one TODO and reformat another
From review.