Add a field borrow_offset to the type descriptor indicating

what amount a T* pointer must be adjusted to reach the contents
of the box. For `~T` types, this requires knowing the type `T`,
which is not known in the case of objects.

src/rt/rust_type.h

@@ -58,6 +58,7 @@ struct type_desc {
     glue_fn *drop_glue;
     glue_fn *free_glue;
     glue_fn *visit_glue;
+    size_t borrow_offset;
 };
 
 extern "C" type_desc *rust_clone_type_desc(type_desc*);

src/rt/rust_util.cpp

@@ -21,6 +21,7 @@ struct type_desc str_body_tydesc = {
     NULL, // drop_glue
     NULL, // free_glue
     NULL, // visit_glue
+    0, // borrow_offset
 };
 
 //