Expose decorated attribute read/write/is_allowed handlers

Closes #765 (closed).

Adds self.my_attr.read(), .write(value) and .is_allowed(request_type) to call an attribute's handlers from device code. The handlers run with the user's own decorators (e.g. DebugIt); other access is delegated to the underlying Attribute. In asyncio mode the calls return awaitables.

Includes tests across all green modes, docstring and news entry.