Implement ITypeInfo#subtype_tag_or_class
Use ITypeInfo#subtype_tag_or_class instead of #subtype_tag
Get rid of the :interface_pointer type tag
Implement ITypeInfo#tag_or_class
Implement ITypeInfo#subtype_tag_or_class using #tag_or_class
Refactor ITypeInfo and InPointer
Implement ITypeInfo#type_or_class_name
Make ITypeInfo#subtype_tag_or_class_name use #tag_or_class_name
Move ISignalInfo#itypeinfo_to_callback_ffitype to ITypeInfo#to_callback_ffitype
Clean up unused code InPointer and InOutPointer

- InOutPointer: Remove sub_type parameter
- InPointer: Remove some unused code
Prepare version 0.6.0 for release