Implemented error messages.

Added mechanism to track errors and implemented for lexer/parser and symbol finding errors.
Moved syntax tree to generated code.

Expanded the code generator to also generate the syntax node tree.
Moved syntax validation to a visitor.

Syntax validation should not be done in the syntax nodes themselves. Moved to a separate visitor.
Implemented binding.

Implemented the bound tree and the binder to build the bound tree from the syntax tree.