Wrap the raw pointers exposed across the bridge as named types that approximate
Swift's OpaquePointer?. Parameterize NativeHandleOwner with the native handle
type.
Convert the *Key and keyPair properties on *PreKeyRecord to throwing
methods.
Since the usual constructor is strongly typed, these should only throw
when the serialized data is corrupted. But that is a possibility.
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.
