Many changes:

- Meta note: I broke lists (and sub-lists) for awhile. It's
  tricky. This commit gets it right (AFIK), so if it breaks again
  compare carefully to this.

- Add `chars-in-balanced` parser.

- Replace some last uses of <!> with notFollowedBy, since lastest
  version of Parsack changed <!>.

- Rename @SPLICE to SPLICE because see following...

- Use #lang at-exp for text fragments.

- $end-line behaves differently based on `list-item-state?` parameter,
  necessary to get tight lists working correctly.

- Sort out things related to use of $label and $link, including
  getting image links working correctly.

- `parse-markdown*` (for internal use) no longer appends "\n", which
  somes uses of it don't want. Up to caller to append if necessary.

- Add the regression tests accumulated from the old regexp parser.