[records] updates for Preview 9. hashCode

[John Rose]

P.S. Another way to deal with toString is just to specify exactly
what we do (it’s not bad!) and follow up with a “subject to change”
message.  We took this tack with MethodHandle.toString:

https://docs.oracle.com/javase/8/docs/api/java/lang/invoke/MethodHandle.html#toString--
> Future releases of this API may add further information to the string representation. Therefore, the present syntax should not be parsed by applications.