API to turn Data into read-only reference

[jackkoenig]

Type of issue: feature request

Impact: API addition (no impact on existing code)

Development Phase: request

What is the use case for changing the behavior?

This is a proposal to have an API to turn any Data object into a read-only reference. You can do this with UInts by performing some no-op (eg. x + 0.U), but there is no way that I can think of to do the same with an Aggregate.

I'm not sure what the API should, maybe .readOnly or ReadOnlyWireInit(...), but it can easily just map to a node in FIRRTL: node _T_1 = originalSignal, const prop will remove the node.