The number 23 is special in the sense that all of its digits are prime numbers.
Furthermore, it's a prime itself. There are 4 such numbers between 10 and 100:
23, 37, 53, 73. Let's call these numbers "total primes".
Complete the function that takes a range (a, b) and returns the number of
total primes within that range (a <= primes < b). The test ranges go up to
10^7.
(10, 100) ==> 4 # 23, 37, 53, 73
(500, 600) ==> 3 # 523, 557, 577