[math-fun] A faster algorithm for π(x)

Earlier, I had the doc π(n) and the Sieve of Eratosthenes <https://docs.google.com/document/d/1yzj1QZZzmqVG4ow6ScTR5bX5mDOKxNVkQGX7G-HAJfQ/edit>, but I have a better algorithm: This is the same method as in the doc (read the doc and edit it) but we don't add 1 because then it will be hard to compute. What do you think of it? It works, I tried it. -Varun