Docx rdr: Avoid mapping makeHeaderAnchors globally

It only applies to headers, so we can just apply it when we make a
header.
Docx Reader: Clean up guards

Use PatternGuards to get rid of need for `isJust`, `fromJust`
altogether.
Docx Reader: Simplify makeHeaderAnchors

Using pattern guard, in preparation for doing some more complicated
stuff with it (recording header anchors, so we can change them to auto
ids.)