Add functions gcd() and lcm() for integer and numeric types.

Add functions gcd() and lcm() for integer and numeric types.

These compute the greatest common divisor and least common multiple of
a pair of numbers using the Euclidean algorithm.

Vik Fearing, reviewed by Fabien Coelho.

Discussion: http://postgr.es/m/adbd3e0b-e3f1-5bbc-21db-03caf1cef0f7@2ndquadrant.com