Add named constants or enums

## Motivation
There are a variety of ways to create `Const`s to represent enumerable types in hardware with ROHD, and currently everyone comes up with their own way that's convenient for them. Creating a standardized way of doing it would improve consistency, encourage best practices, and provide an opportunity for helpful utilities and automation.
- Encoding features could be added.
- Generated outputs (e.g. SV) could contain named constants instead of plain numbers.

## Desired solution
Some type other than `Const` that can be used to represent named constants or enums in hardware.
- Should support options for a variety of encodings (e.g. one-hot, gray, sequential, etc.)
- Should enable generated outputs to use names for constants instead of just directly exporting the numbers.
- e.g. SystemVerilog could use `localparam`s with names for constants in FSM abstractions

## Alternatives considered
Could just leave users to create their own enums and live with constant numbers in generated code (i.e. no change).

## Additional details
This was discussed during FSM abstraction discussions in the ROHD Forum on July 27, 2022. Thanks Eric and Omer for the discussion and ideas!