Implement new normalization.

There were some problems with the old str normalization. This fixes those
problems. Also, since it drills down on its own, it only needs to be
mapped over the blocks, not walked over the tree.
Add normalization test.

Add torture-test for new normalization functions.

One problem that this test demonstrates is that word has a tendency to
turn off formatting at a space, and then turn it back on after. I'm not
sure yet whether this is something we should fix.
Docx reader: Fix spacing in formatting.

The normalizing tests revealed a problem with unformatted spaces, brought about
by `spanTrim`. This fixes by not trimming the spaces out of spans until they
are in their final form.
Docx reader tests: Correct normalize test.
Docx reader tests: add tests for normalization deep in blocks.
Merge pull request #1363 from jkr/newNormalize

Improve normalization