make run function more composable

[debs-sifive]

Could I get this onto 3.3.x so I can use it in rocket chip, please?

Related issue: chipsalliance/rocket-chip#2467

Type of change: feature request

Impact: API addition (no impact on existing code)

Development Phase: implementation

Release Notes

This allows for ChiselStage's run function to be called externally. For example, from RocketStage.

[seldridge]

I've been thinking that Stage.run should likely be a protected method and not exposed publicly. Hence, I'd prefer a solution that avoids further expanding the ChiselStage API. However, I also want to get the underlying issue fixed quickly in Rocket Chip.

Towards this, I have an implementation of an alternative solution in chipsalliance/rocket-chip#2481. Would that be an acceptable solution?

[debs-sifive]

@seldridge your solution makes sense to me!