… & `SecIdentity`

Allows selecting accessibility (i.e. `kSecAttrAccessible`) when generating or saving items.

As a requirement we add `kSecUseDataProtectionKeychain` to keychain requests. This has the benefit of normalizing keychain access across macOS & iOS, tvOS, watchOS); allowing the removal of the last usage of `#if os(macOS)/#endif` when accessing the keychain.

`SecKey.attributes()` now retrieves _all_ attributes from using `SecItemCopyMatching` instead of  `SecKeyCopyAttributes`; this is backward compatible with previous functionality with _more_ attributes. `SecKey.keyAttributes()` can be used to get previous _smaller_ list of attributes (which may also be faster).

…a failure