Safety-proofed all ordinal constant comparisons with '==' instead of 'is'.

Note that some experimentation showed that defining ...

    def __copy__(self, *args):
        return self

... on OrdinalConstant appears to "fix" the comparison discrepancies reported
in an earlier commit.

But it's almost certain that 'is' comparison will continue to fail after
pickling.

CONCLUSION: always use '==' (not 'is') to compare Abjad ordinal constants.
(Just like for instances of all custom classes.)