fix(library/standard/kernel.lean): fix typo
feat(library/standard/int): define le, prove some theorems

Prove interaction of le and add, no interaction with sub, neg, etc. yet
feat(library/standard/int): prove more theorems about le, define mul

Prove most interactions of le with other operators. Define multiplication, and prove that it is compatible with the relation on N ## N
style(library/standard): rename theorems in nat, move int4 to int

Remove int,int2,int3 and move int4 to int. Rename many theorem and definition names, mostly in nat.