Docx reader: Use TeXMath to write math

The new version of TeXMath can translate from its type system into
LaTeX. So instead of writing the LaTeX ourself, we write to the TeXMath
`Exp` type, and let TeXMath do the rest.
Add a note on how `mapD` works.
Remove now-unnecessary TexChar

TeXMath does the work now.
Docx reader: remove now-unnecessary state variable.

This also introduces a `defaultDState` value.
Merge pull request #1491 from jkr/texmath-equations

Docx Reader: Use TeXMath for writing equations.