Simplified link_label parser.

It now just scans for an unescaped `]`.  No `[` characters are
permitted in labels.  Backticks don't have their usual meaning in
labels.

This accords with the behavior of some of the main Markdown parsers:
marked, sundown, discount, kramdown, showdown, Markdown.pl,
PHP Markdown.
Initial steps towards link parsing.
Got inline links working.
Got ref links working, but with deallocation issues.
Fixed allocation issue.
Fixed problem with images.
Fixed backslash-escape inside link label.

Down to 8 failures, all cases where the spec will need to be
changed to reflect lack of priority of links over emphasis.
Disallow links inside links and images inside images.
Removed some unused code.
Added MAX_LINK_LABEL_LENGTH to cmark.h.

Use in link label parsing and reference lookup.
runtests.pl:  distinguish error status from failures.
Restored priority of links over emphasis grouping.

Now when we encounter (possibly) closing `*` or `_` delimiters,
we simply add them to the delimiters stack.

This gets processed by `process_emphasis` either (a) when
a link is created (in which case only the inlines created
by the link are processed) or at the end of processing
a run of inlines.