math: add gcd and lcm

Add gcd and lcm functions to calculate the greatest common divisor (gcd)
and the least common multiple (lcm) to the math module. This will also
work for BigInts that implements its own gcd/lcm.

lib/std/math/bigint.c3

@@ -328,7 +328,7 @@ fn BigInt BigInt.unary_minus(&self)
 }
 
 
-macro void BigInt.div(self, BigInt other)
+macro BigInt BigInt.div(self, BigInt other)
 {
 	self.div_this(other);
 	return self;
@@ -831,6 +831,14 @@ fn BigInt BigInt.gcd(&self, BigInt other)
 	return g;
 }
 
+fn BigInt BigInt.lcm(&self, BigInt other)
+{
+	BigInt x = self.abs();
+	BigInt y = other.abs();
+	BigInt g = y.mult(x);
+	return g.div(x.gcd(y));
+}
+
 <*
  @require bits >> 5 < MAX_LEN "Required bits > maxlength"
 *>