[Spec] Overhaul directive parsing (#247)

* Specify parsing imperatively

This commit overhauls the parsing steps to avoid using the EBNF grammar
for validity, instead specifying that imperatively. It also moves
parsing to happen earlier in the process so that we pass around parsed
Text Directive objects.

Also makes the steps more precise, referring to infra types and
correctly decoding the strings.

Fixes #221
Fixes #230

* Fix and make grammar non-normative

The grammar is now provided solely as a convenience so this makes the
section non-normative. Also fixes it so that UnknownDirective doesn't
subsume TextDirective.

Fixes #220