feat(library/standard/): Develop basic library about integer addition and subtraction, and some minor fixes

Rename the distributivity theorems in nat.lean and nat2.lean.
Separate quotient and int files
Prove most theorems on integer addition, subtraction and negation
fix(library/standard/nat2.lean): redo my changes to nat2.lean

Finished renaming of left/right distributivity
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.
feat(library/standard): wrote makedoc script, created documentation for nat