Docx verbatim code formatting

This adds tests for verbatim code formatting, both inline and in blocks, and then passes those tests. Some of these changes are reimplementations of things that were lost in the rewrite, and a few were things that might never have worked because we never tested for them.

[Note: I also since pushed a test for definition lists, and a bug fix to pass those tests. This should now make good on all the promised in the implementation checklist in the comment block in Docx.hs]

At the moment code works from Docx styles: `VerbatimChar` for inline, and `Code` and `SourceCode` for blocks. These conventions are based on the naming in the Docx writer. But it can easily be supplemented by adding to the lists in `codeSpans` and `codeDivs`.