Change CHECK to BZ.

CHECK instruction was originally intended for '[' of
brainf**k. However, '[' is similar to 'while(*ptr)', which is similer
to 'branch when zero', thus change CHECK instruction to BZ.
Modify the stack so that it is now a generic stack.

The stack now receive a void* as item. Corresponding testing code were
added. An error in stack_push were also found and removed.