Invalid IL on value type instance method

```clj
(let [h (System.HashCode.)]
      (.Add h 1)
      (.ToHashCode h))
;; Exception System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidProgramException: Invalid IL code in <magic>expr_382:eval (): IL_003a: callvirt  0x0a000002
```

we still don't have the full story on value type instance methods locked down