[Tracker] Handshake-to-HW lowering

Nobody has claimed this yet.

This issue intends to track the progress on completing the [handshake-to-hw lowering](https://github.com/llvm/circt/blob/main/include/circt/Conversion/HandshakeToHW.h), and by doing so, transition off of our current [FIRRTL-based lowering path](https://github.com/llvm/circt/blob/main/include/circt/Conversion/HandshakeToFIRRTL.h).

The

* [x] implement top-level lowering c7c07659335580fc531ac58e840be056b8c8fcd8
* [ ] Implement generic handshake operations
* These are your unit-rate actors, muxes, ... - we should have all of the capabilities needed in `hw`, `comb` to elaborate these operations.
* [ ] Determine a memory abstraction
* FIRRTL provides a memory operation that greatly simplifies lowering memories in the current flow. The `seq` dialect does not yet have such memory abstraction. To transition, we therefore need to figure out whether to implement first-class support in the `seq` dialect, or if some other route is needed.
* [ ] Determine a top-level memory interface
* In this new lowering flow, all ground types (data signals) are modeled using ESI channels (ready, valid, data signals). This leaves us with the question of how to expose a memory connection at the top-level (and internally, in the hardware module). Designing around ESI interfaces ideally allows us to compose different hardware modules, and so we'd equally want a memory interface abstraction from ESI.

Start by reading include/circt/Conversion/HandshakeToHW.h and the existing FIRRTL path in include/circt/Conversion/HandshakeToFIRRTL.h. Review the tracker’s unresolved generic-operation and memory-abstraction items; done requires completing the listed lowering work and determining the required memory abstractions and top-level interface.

Written by the indexing model from the issue text.