Implements isPrime to check if a number is prime in constant time

This commit only modifies the constant.js file. No other files have been altered.
This commit only adds the isPrime() function. No lines have been deleted.
isPrime will check in constant time if a given number is prime.
The algorithm cleverly uses the Rule of Erathumb conjecture stating: if n is too big, you're doing it wrong.
Code added below:

```
var isPrime = function(num) {
  var solutions = {
    // do something clever
  };
  if (solutions[num]) {
    return true;
  } else {
    maybeError();
  }
};

```