Take commit 7634029 one step further, with $plain

Although this seems to have negligible speedup, it helps a little and in
any case is consistent with commit 7634029.
Lower performance thresholds.

Back to the levels before commit ec4e45f raised them 5 months ago.
Remove unnecessary `try`s from $setext-heading/para/plain

However not appreciably faster.
Rewrite parser for 4+ `*` or `_`

Simpler and removes one `<or>`. However not appreciably faster.
Simplify $_strong and $_emph

Although this removes some `<or>`s, not appreciably faster.
Delete unused debugging code.
Add page-feed marks for Emacs C-x [ and C-x ] nav

At some point maybe these "pages" should be split into separate source
files. Meanwhile, this makes it easier to jump to the major sections.
Use (in-list)