reorganize grammar and tokens

[drewbanin]

break up lang.lang into a bunch of smaller files which combine the grammar and parsed classes. Eg:

# lang/expressions/case.py

class CaseWhenExpression(BaseExpression):
  @classmethod
  def grammar(self):
    toks = (CASE + ...)
    toks.setParseAction(cls.from_tokens)

  @classmethod
  def from_tokens(cls, string, loc, toks):
    return CaseWhenExpression(...)

Then call grammar() on everything from lang.lang to assemble the full grammar