Ensure self to be a RawData object

[marcgurevitx]

In the current C++ implementation of RawData there's a bug: Methods with self param accept anything that has a _handle, even an open file object.

> f = @RawData.byte
> f file.open("x.ms")  // doesn't raise an error
... ///nonsense

This PR makes it raise an index error (just like in Mini Micro).

(edit: corrected the snippet.)