feat(library/standard/data/quotient): import quotient from lean 0.1
feat(library/standard): port int, and reorganize a lot
fix(lean/library/standard): fix tests, more cleanup
feat(library/standard/data/nat/div): port div
fix(library/unifier): remove incorrect assertion, a local constant may occur in the type of a metavariable that was not instantiated yet

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(doc/lean/tutorial): adjust tutorial to library changes, fix test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>