Add support for marking things as readOnly (backport #4190)

[mergify]

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).

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

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.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

---

This is an automatic backport of pull request #4190 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 7be4243 has failed:

On branch mergify/bp/6.x/pr-4190
Your branch is up to date with 'origin/6.x'.

You are currently cherry-picking commit 7be42430a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   core/src/main/scala/chisel3/Aggregate.scala
	modified:   core/src/main/scala/chisel3/Data.scala
	modified:   core/src/main/scala/chisel3/Element.scala
	modified:   core/src/main/scala/chisel3/experimental/dataview/package.scala
	modified:   core/src/main/scala/chisel3/internal/BiConnect.scala
	modified:   core/src/main/scala/chisel3/internal/Builder.scala
	modified:   core/src/main/scala/chisel3/internal/MonoConnect.scala
	modified:   core/src/main/scala/chisel3/probe/package.scala
	modified:   core/src/main/scala/chisel3/properties/Property.scala
	modified:   src/main/scala/chisel3/simulator/package.scala
	modified:   src/test/scala/chisel3/experimental/dataview/ReifySpec.scala
	new file:   src/test/scala/chiselTests/ReadOnlySpec.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/main/scala/chisel3/experimental/hierarchy/core/Lookupable.scala
	both modified:   core/src/main/scala/chisel3/internal/Binding.scala

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