Make sure public key bit length is an even number

For an odd n-bit number, we can't find two n-1-bit numbers which
multiplied gives an n-bit number. That's why public key length n must be
an even number.

I have also renamed AreSafePrimes to IsSafePrime since only the first
function argument is considered to be a safe prime according to a safe
prime definition.

safe_prime_generator_test.go

@@ -60,7 +60,7 @@ func TestAsyncGenerator(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				AreSafePrimes(p, q, test.bitLen, t)
+				IsSafePrime(p, q, test.bitLen, t)
 			}
 		})
 	}