fix(library/standard/nat2): improved definitions of div and mod
added facts about div and mod
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