D/I Question -- contextual information for Instances

Definition/Instance API allows us to specify a Module once then stamp down copies of it. This works well from Chisel -> FIRRTL, but it's unclear what the mental model is as it goes through FIRRTL.
Currently FIRRTL (specifically SFC) can modify Modules/Instances in the following ways:
- [A] Reset Inference -- change the types of flops inside based on what the reset of an instance is connected to
- [B] Constant Propagation -- eliminate certain inputs, outputs, or internal logic based on what is connected to the instance
- [C] Width inference -- decide the width of internal signals, registers, I/Os based on what is connected to the instance
- [D] Arbitrary annotations -- change pretty much anything based on what annotations are applied to the Instance

For D/I API to be useful, we need to clearly define what is allowed/expected behavior for each of these cases.

My observations are:

# [A] Reset Inference
SFC seems to check that all of the Instances of the Definition will have the same inferred reset type:
https://scastie.scala-lang.org/O1QxHS4nQ8aITDFCB7suhA

The Definition's reset type is ignored.

https://scastie.scala-lang.org/ORaed9rCRRORAapWYiq3wg
and
https://scastie.scala-lang.org/kvx1kLUBS5aHtPHIrmVyQA

# [B] Constant Prop

I *think* that constant prop will happen if it can be constant-propped for all instances, and will not happen otherwise. TODO Scastie example

# [C] Width Inference

Seems to take the maximum width?
https://scastie.scala-lang.org/5Bzl0WCDQ2ewUJ65Z8Hv0Q

# [D] Arbitrary Instance Annotations

Not sure yet

**Type of issue**: feature request

**Impact**: unknown

**Development Phase**: request

**Other information**

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

**What is the current behavior?**
See above

**What is the expected behavior?**

Not sure. My request is that we make sure we are clear on what the expected behavior is.

**Please tell us about your environment:**
- version: 3.5.1

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

More useful Definition/Instance API