Rewrite Docx.hs and Reducible to use Builder.

Can we start setting things on fire now that 1.13 is out? Cool.

The big news here is a rewrite of Docx to use the builder
functions. As opposed to previous attempts, we now see a significant
speedup -- times are cut in half (or more) in a few informal tests.

Reducible has also been rewritten. It can doubtless be simplified and
clarified further. We can consider this, at the moment, a reference for
correct behavior for Docx's usage.