fix: enforce simple form uniqueness in ContinuedFraction.extend op

[sr-murthy]

Enforce simple form uniqueness (of sequences of elements) in ContinuedFraction``extend operation - the current implementation allows the construction of instances with sequences of elements with the terminal element being 1, violating uniqueness. Not a bug as op constructs the correct fractions in extensions, but anomaly in the uniquness of sequence of elements needs fixing.

>>> cf = ContinuedFraction.from_elements(3, 4)
>>> cf
ContinuedFraction(13, 4)
>>> cf.extend(1)
>>> cf
ContinuedFraction(16, 5)
>>> cf.elements
(3, 4, 1)
>>> ContinuedFraction.from_elements(3, 5) == ContinuedFraction.from_elements(3, 4, 1)
True
>>> cf = ContinuedFraction.from_elements(13, 4)
>>> cf
ContinuedFraction(13, 4)
>>> cf.extend(2, 1)
>>> cf
ContinuedFraction(42, 13)
>>> cf.elements
(3, 4, 2, 1)
>>> ContinuedFraction.from_elements(3, 4, 3) == ContinuedFraction.from_elements(3, 4, 2, 1)
True

For the tail extension (3 4) ~> (3, 4, 1) the resulting instance ContinuedFraction(16, 5) is correct as a fraction but as a unique simple continued fraction should have the sequence of elements 3, 5 rather than the sequence 3, 4, 1, as [3; 5] = [3; 4 + 1] = [3; 4, 1]. Same for the tail extension (3, 4) ~> (3, 4, 2, 1): the resulting instance ContinuedFraction(42, 13) is correct as a fraction but as a unique simple continued fraction should have the sequence of elements 3, 4, 3 rather than the sequence 3, 4, 2, 1, as [3; 4, 3] = [3; 4, 2 + 1] = [3; 4, 2, 1].