* app.d: Tweaked code to only grind the grammar through Pegged when
  it has been modified. This lowers compilation time, but
  unfortunately requires two compiles with "dub" to work. Once we move
  beyond parsing, it should significantly speed up compilation, since
  we won't be working on the grammar anymore. At that point only a
  single compilation will be sufficient as well.

* grammar.d: Grammar is now only a storage file.