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