Fix computation of stack depth for classdef and closures.

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.