elang/compiler: Introduce |TokenType::RightAngleBracket| for ease of type argument list and type paramter list easier. This patch also add "?." as |TokenType::NullDor|.
elang/compiler: Introduce |ast::Field| along with type parser.
elang/compiler: Introduce |operator==(Token*, TokenType)| for ease of writing parser.
elang/compiler: Introduce class |Modifier| and |ModifiersBuilder| and utilize them in parser and AST.