Issue #16286: write a new subfunction bytes_compare_eq()

* cleanup bytes_richcompare()
* PyUnicode_RichCompare(): replace a test with a XOR
Issue #16286: optimize PyUnicode_RichCompare() for identical strings (same
pointer) for any operator, not only Py_EQ and Py_NE.

Code of bytes_richcompare() and PyUnicode_RichCompare() is now closer.
Issue #16286: remove duplicated identity check from unicode_compare()

Move the test to PyUnicode_Compare()
Issue #19424: PyUnicode_CompareWithASCIIString() normalizes memcmp() result
to -1, 0, 1