moductypes: Make sure we can apply .sizeof() to all aggregate types.

Before, sizeof() could be applied to a structure field only if that field
was itself a structure. Now it can be applied to PTR and ARRAY fields too.
It's not possible to apply it to scalar fields though, because as soon as
scalar field (int or float) is dereferenced, its value is converted into
Python int/float value, and all original type info is lost. Moreover, we
allow sizeof of type definitions too, and there int is used to represent
(scalar) types. So, we have ambiguity what int may be - either dereferenced
scalar structure field, or encoded scalar type. So, rather throw an error
if user tries to apply sizeof() to int.