AST: Expressions and Statements

* 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
Parser: Check AST in statements test cases

* Fix parser to make them pass
* Fix TypeConstructor signature. While we're at it, replace with
  TypeArray class, as discussed in chat
* reorder test cases by grammar section
Implement and test expressions

* Separate NewArrayExpr and NewObjectExpr
* Assignment is not a Statement
* Banish ListBuffer from SyntaxTree
Merge pull request #85 from Kha/ast

Implement remaining parser, fix AST where necessary