Move Canonical Proto-Source to `Vlsir`

[Dan Fritchman]

Layout21 is part of a family of projects, largely built around and driven by their Protobuf-defined data schemas.
The canonical home for that schema is now https://github.com/dan-fritchman/Vlsir.

Layout21 thus far has a "vendored" (and likely slowly becoming incompatible) version of this schema.
Moving it over will (I think) incur:

• Adding Rust bindings to the Vlsir repository.
• Moving to depend on them, generally deprecating layout21protos
• Cleaning up whatever odds & ends follow

[Dan Fritchman]

PR #9 is in to address this.