Primes sequence

[Bodigrim]

This PR is inspired by forprime control structure in PARI/GP and provides an Enum instance for Prime.

One of the most ubiquitous tasks in number theory is generating lists of primes. Current API of arithmoi (represented by Math.NumberTheory.Primes.Sieve) is sufficient, but burdens users with too many choices. Should I use primes or sieveFrom or primeList? Or maybe it is faster just to filter a short interval with isPrime and do not bother with sieves? Frankly speaking, even I was never able to remember all subtleties.

Well, Haskell has a well established and handy notation for list generation and enumeratable data. And now it is available for primes:

• Generate primes from the given interval:

  > [nextPrime 101 .. precPrime 150]
  [Prime 101,Prime 103,Prime 107,Prime 109,Prime 113,Prime 127,Prime 131,Prime 137,Prime 139,Prime 149]

• Generate infinite list of primes:

  > [nextPrime 101 ..]
  [Prime 101,Prime 103,Prime 107,Prime 109,Prime 113,Prime 127,Prime 131,Prime 137,Prime 139,Prime 149...

• Generate primes from the given interval of form p = 6k+5:

  > [nextPrime 101, nextPrime 107 .. precPrime 150]
  [Prime 101,Prime 107,Prime 113,Prime 131,Prime 137,Prime 149]

• Get next prime:

  > succ (nextPrime 101)
  Prime 103

• Get previous prime:

  > prec (nextPrime 101)
  Prime 97

• Count primes less than a given number:

  > fromEnum (precPrime 100)
  25

• Get 25-th prime number:

  > toEnum 25 :: Prime Int 
  Prime 97

I wanted to implement this API for ages and first drafts are at least two years old. But it took a lot of time to mature.

[Bodigrim]

Now it is ready.

@rockbmb @b-mehta @CarlEdman could someone please review?