fix Can't call function on object that is also a method #821

[asukaminato0721]

fix #821

Bound-method attribute resolution now also consults the wrapped function type before giving up, so descriptors that attach helpers like clear_cache remain visible after binding; this is handled by expanding the Type::BoundMethod branch to recurse into func.as_type() alongside the existing types.MethodType lookup.

[stroxler]

This looks like a great start, thanks!

I think there's one issue here which is that we're abusing a data structure intended to track unions - the vec of AtrributeBase1 in AttributeBase, to do something pretty different.

I think we need to handle this case with a single AttributeBase1 instead of two of them to maintain code simplicity.

One option that might work would be to just make BoundMethodFunction attribute lookup fall back to self.stdlib.method_type().clone() at attribute lookup time for not-found attributes.

But actually I think there's another option, which is to take a peek at the bound_method type that's inside Type::BoundMethod(bound_method)... the self.stdlib.method_type().clone() is really specifically for built-in method descriptors, whereas the case we're handling here is a custom descriptor. So we probably could just use the actual type bound_method.func when it isn't Callable or types.FunctionType.