Implement polynomial multiplication in the canonical basis.

Polynomial-polynomial multiplication in the canonical basis is now supported.

Additionally:

• The test suite has been updated to check the behavior of the new implementation.
• Unify operands verification procedure for multiplication and addition operations.
• Introduce is_constant() high-level utility function that returns True if a polynomial instance (in any basis) is a constant initialized polynomial; it returns False otherwise.

This commit should resolve Issue #163 (closed) and part of extending the support for arithmetic operations involving polynomial (see Issue #142 (closed)).