Adds OrgError.nestingTooDeep and the depth limit behind it: input nesting past
OrgParser.nestingLimit is refused instead of overflowing the stack. Replaces a
measured crash, not a theorised one.

Breaking: OrgError gains a case, so an exhaustive switch needs a new branch.

Also: parseOrg's documentation now describes the parser that exists, and every
gate runs on push.

See CHANGELOG.md.