Prime number algorithms

[djtrack16]

isPrime returns true if a positive integer is prime, false otherwise.

'isCoprime` returns true if the GCD of two positive integers is 1, false otherwise.

totient returns the value of Euler's totient function.

primeFactors returns a list of the prime factors of a given positive integer.

primeFactorsMultiplicity returns a list of tuples that relays the multiplicity of the prime factors.

goldbachNumbers returns the pair of prime numbers that sum up to a given even number, i.e. Goldbach's conjecture.

goldbachCompositions returns a list of tuples that, given an upper and lower bound of positive integers, is every even numbers within that bound and their goldbach numbers.

This is the draft of the PR. If it is OK so far, then I will write tests for all these functions to complete the PR.

Suggestions welcome.