Fix int * Size operation and add tests (#1158792)

When Size is not on the left it needs to call the correct Decimal method
and make sure it returns a Size object. This also adds a test to make
sure the results of add, subtract, multiply, divide, modulo and power
return the current behavior.

(cherry picked from commit fa886a67fbd1099a6a8f6e88aa7ab6584d4f4ce6)