chore(examples/lean): use macros in examples

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/elaborator): process ho-match before normalizing

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/Nat): add basic axioms and theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/Nat): cleanup and add PlusAssoc

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/Nat): cleanup and add PlusAssoc