Declare all loaders to receive an object supporting the buffer interface

Declare all loaders to receive an object supporting the buffer interface

There isn't in mypy such an object, so just use `Union[bytes, bytearray,
memoryview]`

Avoid a memory copy passing data from the Transformer to the loaders.

I had started exercising this code path by adding copy tests, but after
dropping the extra copy the same path is now exercised by any select, so
I've stopped doing that.