Various changes to formal spec.

- Support format references. This adds a 'prelude' environment
  containing format definitions to various judgements.

- Support arrays sized by bytes as well as by number of elements.

- Separate out various forms of tau to hopefully clarify and
  simplify the typing rules.

- Add a round() function to help with size computations,
  typically used for sized arrays.

- Add a 'parse within' construct that keeps track of a byte-size
  limit when parsing a format.  This doesn't affect typing, but does
  affect code-generation.
Remove labels from ott spec (previously marked as a todo).
Minor cleanup in omakefile.
Remove label construct from implementation.
Update copyright year in headache conf.
Minor cleanup in lexer.
Remove obsolete comment.
Add support for named formats as array and branch values; disable codegen pending reworking it.
Minor updates to notes/todo.