Integrating Semantic Analyzer

- ASTNode append auto-correction reinstalled.

- Finished Semantic Analyzer, integrated privately
  inside the Parser module.

- Corrected a bug in Parser.parseDiv() that
  produces extra DIV elements if it contains
  no visible blocks.

- Refactored some constants to use all
  capital letters. Namely, LexToken.types
  and ASTNode.types to LexToken.ENUM and
  ASTNode.ENUM, respectively.

- Shortened ParserBase.lookAhead to .peek().