Docx reader: simplify blockNormalize

Use a function `stripSpaces`, instead of recursion. Makes it a bit
easier to read and mantain, and simplify normalizing DefinitionList,
which was left out the first time.
Docx Reader: Normalize DefinitionLists

Previously DefinitionList had been left out of `blockNormalize`. Now it
is included.
Docx reader: Add a comment explaining strNormalize

`normalize` from Text.Pandoc.Shared is more general. In tests, though,
it more than doubles the run time. `strNormalize` does less, but it does
what we need. This comment is added for future maintainability.