Parse "w:sym"

From @mpickering. Deal with `<w:sym>` elements. These are difficult because they make reference to a codepoint in a specified font. Right now, we include data for dealing with the Symbol font. A lookup table is generated from an included unicode document on the Symbol font, placed in lib.

I am interested to know if any other fonts make their way into symbol-heavy docx documents? Some corpus of documents would be helpful here -- perhaps someone teaching an intro logic course who takes homework in docx? Economists? In any case, if we had a corpus, we could run the following:

~~~{.sh}
for i in *.docx; do
    unzip -p $i "word/document.xml" | tidy -xml -q | grep "w:sym"
done
~~~

to get a better idea.