[HW] Move off of mlir::ModuleOp to hw::DesignOp (or something)

MLIR upstream is getting progressively better support for the top level op being something other than `mlir::ModuleOp`. Given that I think we should introduce a new `hw.design` op (better suggestions welcome) to replace this.

This is mostly an idealistic cleanup, but I think it makes sense for a couple of reasons:

1) It would eliminate the confusing and annoying `ModuleOp` vs `HWModuleOp` divide.
2) It would reduce dependencies on the "mlir/IR/BuiltinOps.h" from various places.
3) It gets CIRCT into a more controlled model, where hw::ModuleOp is tied to hw::DesignOp

-Chris

Start by reviewing the MLIR top-level operation support and the cited mlir/IR/BuiltinOps.h dependency. Map where mlir::ModuleOp and hw::ModuleOp are used, then define the replacement design and verify that the confusing divide is removed, dependencies are reduced, and hw::ModuleOp is tied to the new design operation.

Written by the indexing model from the issue text.