Maybe Make the VLSIR Dependency Optional

[dan-fritchman]

We have a use case which:

• Does not (we think) need any Hdl21 <-> VLSIR conversions
• Needs a different version of protobuf, which conflicts with VLSIR's

The few options seem to be:

• Make Hdl21 not necessarily depend on VLSIR, i.e. have an optional "feature flag" (definitely on by default) for it
• Somehow peel the parts this use-case needs out of Hdl21?