feat(library/standard/int): start on integers file
define integers, addition. int.lean is an old version, int2.lean is a new try which works better
int2.lean fails to compile when L169 is uncommented
feat(library/standard/list): add append definition, and related theorems
feat(library/proof_style): add documentation of proof style
feat(library/standard/nat2): definition of div and mod
feat(library/standard): change positivity in nat, change definition of add in int

Change the positivity side conditions in nat (needed for some multiplication lemmas) to _ > 0 instead of being of the form succ _. In int added sub : nat -> nat -> int