elang/compiler: Introduce |Parser::ConsumeTokenIf(TokenType)|.
elang/compiler: Introduce |ast::Assignment|, |ast::BinaryOperation|, |ast::Conditional|, |ast::Literal|, |ast::NameReference| and |ast::UnaryOperation|
elang/compiler/testing: Introduce |ast::Enum| formatter for testing.
elang/compiler: Introduce |partial| keyword.
elang/compiler: Introduce |ast::Visitor|.
elang/compiler/testing: Utilize |ast::Visitor| in |Formatter|.