Continuing 5dbc8a1b (decoder: Don't allow mark to be returned as pickle
result) I discovered that the mark object can be still exposed to user,
but not directly. For example the following pickle:

	"(\x85." // MARK + TUPLE1

was creating Tuple{mark} and returning it just ok to the user.

As marker must be used only internally it is invalid to do so. Python
also forbids this:

        In [3]: s = b"(\x85."

        In [4]: dis(s)
            0: (    MARK
            1: \x85     TUPLE1
            2: .        STOP
        highest protocol among opcodes = 2

        In [5]: pickle.loads(s)
        ---------------------------------------------------------------------------
        UnpicklingError                           Traceback (most recent call last)
        <ipython-input-5-764e4625bc41> in <module>()
        ----> 1 pickle.loads(s)

        UnpicklingError: unexpected MARK found

So let's close all (hopefully) holes where mark object could be returned to
user in a similar way.