make cancel canonical for a given expression

[smichr]

References to other Issues or PRs

fixes #27096

Brief description of what is fixed or changed

The result of p.cancel(q, include=False) should be the same as P.cancel(Q) if (p/q).equals(P/Q). The domain will match the unified domain of p and q, however, so may be different.

Also, any extractable Rational context will be reported; there will never be a distributed constant in numerator or denominator expressions.

Other comments

Release Notes

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