Add support for marking things as readOnly

[jackkoenig]

Also add internal support for marking things as "deprecated read only". This will promptly be used to mark the return value of .asTypeOf as "deprecated read only" in 6.x in preparation of #4168.

I implemented this as part of the view binding for the following reasons:

1. While one could mutate the underlying Data, there are use cases where an API may want to return a read-only reference/view of their underlying Data while reserving the right to connect to it themselves (I intend to do this in proprietary code)
2. We could implement this as a separate Binding (i.e. not part of the existing Views) or as an Arg. Either approach could certainly work, but it would need to compose with views anyway. Consider some of the tests. Given some Data that is marked readOnly that is then combined with other (potentially not read-only) Data to be viewed as some Aggregate type--the read-only aspect of that target needs to be preserved. This "writeability" is thus an intrinsic property of reification anyway, so we might as well just embrace it and make it part of the API so that callers handle it as appropriate (which is what I implemented).

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Type of Improvement

• Feature (or new API)

Desired Merge Strategy

• Squash

Release Notes

Users can call .readOnly on any Data to prevent connections to the returned value. Resolves #1267.

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[jackkoenig]

I'm going to peel off the bit that actually marks the return value as "deprecated read-only" into its own PR.

[jackkoenig]

The diff here can probably be made lower touch. The issueConnect functions call .lref so we could probably rely on the error reporting there, although that means it would construct wires in the error case (to provide a legal lref for a read-only thing) whereas doing it at this level allows us to elide issuing the connection altogether.

Separate from that, this code is also pretty inelegant in that it's checking .topBinding for each left and right multiple times. This may be optimized by the JIT, but it's pretty inefficient if it isn't.

[seldridge]

This approach sounds good to me. I don't have time to do a thorough review today, so please bug me for a more detailed post-merge review.