STRUCT_OBJ is never used.

[skx]

The STRUCT_OBJ type, defined here is never used.

It seems that an instantiated struct describes itself as being a HASH_OBJect here:

func (self *StructInstance) Type() ObjectType { return HASH_OBJ }

Nowhere in the code is STRUCT_OBJ referenced, so it might be as well to remove it - especially given the comment above!