Value type hash code

[Dan Smith]

> On Apr 10, 2018, at 8:49 AM, David Simms <david.simms at oracle.com> wrote:
> 
> * Values containing references whose contents are mutable pose a
>   problem, their hash code is only as stable as the contents of the
>   whole object graph.

There's some flexibility in how we name operations, but let's observe that there are two pairs of operations to consider:

substitutable/substitutableHash: recur on value fields, use acmp on reference fields
deepEquals/deepHash: recur on value fields, use 'equals' on reference fields

I think my expectation has been that substitutable/substitutableHash would be the default behavior of equals/hashCode, with the capability for users to override this if they're confident it's safe. (And with the ability to fall back to the default behavior with System.identityHashCode or something similar.)

—Dan