borrow checker: add method to check that its empty

crates/wiggle/src/borrow.rs

@@ -25,6 +25,9 @@ impl BorrowChecker {
     pub fn is_borrowed(&self, r: Region) -> bool {
         self.bc.borrow().is_borrowed(r)
     }
+    pub fn is_empty(&self) -> bool {
+        self.bc.borrow().is_empty()
+    }
 }
 
 #[derive(Debug)]
@@ -41,6 +44,10 @@ impl InnerBorrowChecker {
         }
     }
 
+    fn is_empty(&self) -> bool {
+        self.borrows.is_empty()
+    }
+
     fn is_borrowed(&self, r: Region) -> bool {
         !self.borrows.values().all(|b| !b.overlaps(r))
     }