Rename expr.y -> parser.y
Modify lexer to ready to recognize block.
Add yydebug to interpreter's main.
Modify syntax (actions are needed to be fixed.)

* Modify syntax file
* Add interface to AST.h (not implemented them)
Implement function definition/calling.
Modify List iteration interface.
Delete dSYM directory on `make clean`.
Remove debug messages.
Fix typo.
Error on invalid symbol reference.
Add stack.
Rename IterationState to ListIterationState.
Add List.size, is_empty.
Add const qualifier to ListIter_current
Implement calling function.