[WIP] Implement remaining parser, fix AST where necessary

* Remove unused Parameters class
* Separate expressions and statements
  * Add ExpressionStatement class. We _could_ let Expression inherit from Statement,
    but I don't think there will be many common code paths.
  * Rename EmptyTree to EmptyStatement. I'm hesitant to include an EmptyExpression,
    so replacing with Option[Expression] instead.
* Add missing ReturnStatement
* Fix signatures of Block, If, While, Apply, Select
* Fixup/implement affected Parser parts