Rename Text.Pandoc.XMLParser -> Text.Pandoc.XML.Light...

..and add new definitions isomorphic to xml-light's, but with
Text instead of String.  This allows us to keep most of the code in
existing readers that use xml-light, but avoid lots of unnecessary
allocation.

We also add versions of the functions from xml-light's
Text.XML.Light.Output and Text.XML.Light.Proc that operate
on our modified XML types, and functions that convert
xml-light types to our types (since some of our dependencies,
like texmath, use xml-light).

Update golden tests for docx and pptx.

OOXML test: Use `showContent` instead of `ppContent` in `displayDiff`.

Docx: Do a manual traversal to unwrap sdt and smartTag.
This is faster, and needed to pass the tests.

Benchmarks:

A = prior to 8ca191604dcd13af27c11d2da225da646ebce6fc (Feb 8)
B = as of 8ca191604dcd13af27c11d2da225da646ebce6fc (Feb 8)
C = this commit

| Reader  |  A    | B      | C     |
| ------- | ----- | ------ | ----- |
| docbook | 18 ms | 12 ms  | 10 ms |
| opml    | 65 ms | 62 ms  | 35 ms |
| jats    | 15 ms | 11 ms  |  9 ms |
| docx    | 72 ms | 69 ms  | 44 ms |
| odt     | 78 ms | 41 ms  | 28 ms |
| epub    | 64 ms | 61 ms  | 56 ms |
| fb2     | 14 ms | 5  ms  | 4 ms  |

1 files changed, 0 insertions, 0 deletions

diff --git a/test/pptx/images.pptx b/test/pptx/images.pptx
index e73126376..670a825de 100644
--- a/test/pptx/images.pptx
+++ b/test/pptx/images.pptx