Description: Define the first Chebyshev function, which adds up the logarithms of all
primes less than x , see definition in ApostolNT p. 75. The
symbol used to represent this function is sometimes the variant greek
letter theta shown here and sometimes the greek letter psi, ψ;
however, this notation can also refer to the second Chebyshev function,
which adds up the logarithms of prime powers instead, see df-chp .
See https://en.wikipedia.org/wiki/Chebyshev_function for a discussion
of the two functions. (Contributed by Mario Carneiro, 15-Sep-2014)