Docx reader: Fix spacing issue.

Previously spaces at the beginning of Emph/Strong/etc were kept
inside. This makes sure they are moved out.
It does in general, but the larger list folding function that uses it
fails if the leading space is in the first element, or the trailing
element is in the last element. I see how to fix this (and throw in some
pattern guards to get rid of the heads and lasts) but I won't get to it
until later today.

I'll also try to type up some comments explaining the purpose of
Reducible. Basically, it's an abstraction on the old span- and
div-combining I used to have to do, due to the fact that docx runs
aren't nested, and we can't be sure of the order or the formatting
descriptions. ([Emph [Str "x], Strong [Emph [Str "x"]]]).

(Container is just a way to test equality of pandoc formatters---Emph,
Strong, etc---and move them around, the way we used to with spans and
divs. This lets us do it on the fly, instead of building up a bunch of
spans and divs, and then walking through them at the end.)