Fix computation of stack depth for classdef and closures. Also minor tweaks to internal routines. Use PyCF_MASK instead of explicit list of flags. For the MAKE_CLOSURE opcode, the number of items popped off the stack depends on both the oparg and the number of free variables for the code object. Fix the code so it accounts for the free variables. In com_classdef(), record an extra pop to account for the STORE call after the BUILD_CLASS. Get rid of some commented out debugging code in com_push() and com_pop(). Factor string resize logic into helper routine com_check_size(). In com_addbyte(), remove redudant if statement after assert. (They test the same condition.) In several routines, use string macros instead of string functions.