feat(library/standard): add more basic theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(shell/lean): only save .olean file if there are no errors

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug in process_eq_constraint

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/logic): add exists unique

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/logic): add exists unique