Change “auxiliary” to “data”

This change prepares for adding support for other nodes with
data/attributes than those created via footnotes (image with alternate
text is one).
Add alternate text support for images
Rename “ref” and “reference” to “link”
Fix location tracking for figures and codeblocks
Improve error messages

Substitute “missing” with “expected” and simplify and clarify the
messages.
Fix bug in token_name_unescape
Use parser location for nmc_parser_oom_error location
Add names to more tokens
Implement u_width()
Update and fix location tracking to use u_width
Improve error messages for footnotes
Remove unnecessary collapse of column values
Restore support for YYLTYPE printing
Improve location tracking code use
Support more variations of links
Update use of link footnotes
Mention that juxtaposed code inlines are merged
Clarify when footnotes are checked/cleared
Change types to names
Use textual names for nodes when debugging