API to instantiate previously-defined Definitions.

**Type of issue**: feature request

**Impact**: API addition (no impact on existing code)

**Development Phase**: proposal

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**

**What is the expected behavior?**

**Please tell us about your environment:**

**What is the use case for changing the behavior?**

This issue is to track future work for https://github.com/chipsalliance/chisel3/pull/2512 to create an API that can instantiate a previously-defined Definition.