Simplify dual

[cchalmers]

Don't merge yet. This is a rewrite of the DUAL tree structure.

Changes:

• Now separated into DALTree and DUALTree. The DALTree is non-empty (except for an empty Concat) and does not contain any u annotations. The DUALTree is either EmptyDUAL or a u annotation with a DALTree. The structure is also mostly strict, I still need to play with things to see what gives the best performance.
• Having the u annotation exclusively at the top simplifies things (lower u annotations where never used before anyway). Changes to the u no longer need to propagate does the whole tree.
• Folding has been simplified a little by enforcing the result is a Monoid.
• There's two variants for folding a dual tree: One is the simple version where all down annotations are pushed to the leafs. The primed version allows you to use "partial" down annotations that are reset between Concats. In diagrams this lets you apply, for example, a clipping to multiple diagrams at once.

[jeffreyrosenbluth]

I've given this and the related pull requests an initial read through and things look good to me. I do think we could use some more comments in the new code.

[byorgey]

Why did you remove the other-extensions?

[cchalmers]

I can't actually remember. I'll add it back.

[byorgey]

OK. Of course the specific list of extensions used has probably changed.

[byorgey]

Let me know what you think of the updated comments I just pushed.

[cchalmers]

Comments look good.