`cmark`’s pretty-printer matches our output pretty well, with a few differences:
- it puts a space between the fence and the info string for in code blocks;
- it prefers `-` over `*` for bulleted lists (as do I) and it indents them;
- it `\`-escapes certain chars very conservatively;
- it prefers indented/unfenced code blocks if there is no info string; and
- it prefers `*` over `_` (unlike any sane person).
This also shows how the change fixes a number of issues:
- fix2158-1.output.md also illustrates how this change fixes#1809;
- alias-many.output.md and input-parse-errors.output.md show how fenced
code blocks without an info string would use the beginning of the
content as the info string;
- transcripts-round-trip/main.output.md shows how output blocks for
generated `unison` stanzas (which could contain nested fenced blocks)
might not have long-enough fences; and
- error-messages.output.md and generic-parse-errors.output.md show how
Unison errors were reported on the wrong line number (and thus the
printed error lines were also incorrect).
Almost everywhere we produce a `Reference` for numbered args, we also have a
`HashQualified Name` handy, which is much more consumable by commands.
The only case we don’t have an `HQ` is in the `todo` command output, so that now explicitly builds
a `HQ.HashOnly`.
This also fixes an issue with `StructuredArgument` handling where `alias.term`
and `alias.type` wouldn’t make an alias to a `HQ.HashOnly` `StructuredArgument`.
Fixes#4898.
