Wrap the raw pointers exposed across the bridge as named types that approximate
Swift's OpaquePointer?. Parameterize NativeHandleOwner with the native handle
type.
getPreKeyId was returning an incorrect value, and the Kyber fields
were missing altogether. None of this affected security of the iOS app
because the iOS app only constructed PreKeyBundles for libsignal to
process, and never read any of these fields. If it did, it would have
failed to establish a session due to the incorrect pre-key ID.
Caught by @dtimoshenko99; thank you!
At this point, the only special behavior of bridge_fn_buffer is to
support multiple return values for the C bridge (a pointer/length
pair), and that doesn't pull its weight. Remove it in favor of a plain
bridge_fn.
This did reveal that Username_Hash was using bridge_fn_buffer and now
produces a fixed-size array, imported into Swift as a tuple, so this
commit also factors out a new helper invokeFnReturningFixedLengthArray.
