Still need one of the sorts
Deduplicate directly in immutable array
Ensure that sorting invariants are respected

This allows to remove a redundant sort at the start of deduplicate.
mul: Pre-sort slices prior to expansion

This shouldn't be necessary, but 'mul' is being handed matrices that are
not properly sorted. Need to codify invariants.
Export everything from library
Test invariants after mappend, add, and mul
Test reorder invariants