The s*/y* value formatters receive a Py_buffer instead of a char *.
This value format is more flexible in the types that it allows.
We change bdiff() to accept any object that conforms to the buffer
protocol. We validate the buffers are contiguous and have a single
dimension.
This allows memoryview instances to be handled by the function, so
we revert a recent change to cast arguments to bytes before calling
this function.