chore(util/sexpr/options): typos

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add markdown documentation
feat(library/standard/logic/axioms): add import default
feat(library/standard/data/nat): port most recent nat to lean 0.2
refactor(library/standard/data/nat): stylistic changes
fix(library/coercion): spurious 'replacing coercion', fixes #22

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