document BorrowChecker, make creation unsafe

crates/wiggle/tests/handles.rs

@@ -34,7 +34,7 @@ impl HandleExercise {
     pub fn test(&self) {
         let ctx = WasiCtx::new();
         let host_memory = HostMemory::new();
-        let bc = BorrowChecker::new();
+        let bc = unsafe { BorrowChecker::new() };
 
         let e = handle_examples::fd_create(&ctx, &host_memory, &bc, self.return_loc.ptr as i32);