[FIRRTL] Add fields to domains

[seldridge]

Extend domains to allow for them to contain fields. These fields provide
a schema of information that all domains of certain kind must have. This
models information that a user must provide for an input domain or is
provided for an output domain.

Opt for a "struct-like" representation as opposed to using input/output
ports like FIRRTL classes. That said, these are intended to be lowered to
classes in a future patch.

[dtzSiFive]

LGTM!
It might make sense to limit the field to a Property-type TypeAttr, ODS or verifier, since looks like at the FIRRTL text level that's an error to not be a property type.

If the field names must be unique (they ofc "must" if indexing is string-based?) consider verifying that as well.

Nice focused incremental addition!

[seldridge]

It might make sense to limit the field to a Property-type TypeAttr, ODS or verifier, since looks like at the FIRRTL text level that's an error to not be a property type.

Good point. I started down this path and couldn't sort through the ODS / header inclusion to get it to compile.

If the field names must be unique (they ofc "must" if indexing is string-based?) consider verifying that as well.

Great idea. I'll add this in a follow-up.

[seldridge]

@dtzSiFive wrote:

It might make sense to limit the field to a Property-type TypeAttr, ODS or verifier, since looks like at the FIRRTL text level that's an error to not be a property type.

PR here: #9094 The primary review request on #9094 is on the reorganization of the ODS files and headers.