(19): greatly extend divisibility and primes
(19): greatly extend divisibility and primes

This includes gcd, Bezout's theorem, Euclidean algorithm, definition of primes, division properties of primes, there are infinite primes, fundamental theorem of arithmetic, multiplicative inverses, the fact that \sqrt{2} is irrational

rlewis1988/logic_and_proof,fpvandoorn/logic_and_proof,fpvandoorn/logic_and_proof,rlewis1988/logic_and_proof,avigad/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,avigad/logic_and_proof,fpvandoorn/logic_and_proof,leanprover/logic_and_proof,rlewis1988/logic_and_proof,leanprover/logic_and_proof