Proposal: ability to detect set/unset fields by client

[boukeversteegh]

This idea has been brought up a few times.

Clients want to know whether an object they've created has a certain field explicitly set or not (even when it was set to the default).

This is different from detecting whether the field was set on the wire, since it is not distinguishable whether a default value was set, or that it was omitted.

We would need this feature internally to fix #74, and for #95.

A discussion about this is available on our Slack at #unset-field-detection. In due time, the content there should be consolidated here on github.