Write a function named `gcd`

that accepts two integers as parameters and returns the greatest common divisor of the two numbers.
The greatest common divisor (GCD) of two integers *a* and *b* is the largest integer that is a factor of both *a* and *b*.
One efficient way to compute the GCD of two numbers is to use Euclid's algorithm, which states the following:

GCD(*A*, *B*) = GCD(*B*, *A* % *B*)

GCD(*A*, 0) = Absolute value of *A*

In other words, if you repeatedly mod *A* by *B* and then swap the two values, eventually *B* will store 0 and *A* will store the greatest common divisor.

For example: `gcd(24, 84)`

returns 12, `gcd(105, 45)`

returns 15, and `gcd(0, 8)`

returns 8.