Clean-up: docs, error handling, old cruft.

Factor now doesn't fail.
Add the `estimate_prime_pi` function.
Implement double-ended iterator.
Use estimate_prime_pi to give a more precise size_hint.
Add "sporadic" prime_pi tests, testing powers of 10 upto 10^17.
Implement estimate_nth_prime & improve prime_pi with Dusart 1998.

Also, convert estimate_prime_pi to take u64. This adds some small
tables (< 2KB).
Add the table generator.
Update & tweak README.

More functionality, yay!