Simplify spec by adding safe math functions

[vbuterin]

The two that immediately come to mind, with multiple use cases already in the spec, are:

def safe_modulo_subtract(a, b, modulus):
    return (a + modulus - b) % modulus

def safe_a_mul_b_div_c(a, b, c, increment):
    return (a // increment) * b // (c // increment)