What should `NoConn`s be able to do?

[dan-fritchman]

"Policy" questions on what the no-connect Signal-type should and should not allow.

The initial implementation from #7 has:

• (a) "Strict" single-port, no other connection to anything requirements
• (b) Connects to Signals, not Bundles

Questions and proposals:

• Should they be allowed to connect non-unity-width ports?
  • Propose yes. This is part of #33.
• Should they be allowed to connect to Bundle valued ports?
  • Propose yes. This is part of #33.
• Should they be allowed in Concat?
  • Propose yes
  • I suppose the weird part would be the inference of widths, given the answers above. The most clear, I guess, would be inferring each NoConn as unit-width(?).
  • This may be a "no for now", just make a Signal instead, especially for non-unit widths.
• Should they be allowed in AnonymousBundle?
  • Propose yes
• Should they be Slice-able?
  • Propose no
• Should stuff like the test in #7 work, or fail:

@h.module 
class Inner:
  p = h.Port()

@h.module
class Bad:
  # Create two instances of `Inner`
  i1 = Inner() 
  i2 = Inner()

  # Connect the first to a `NoConn`
  i1.p = h.NoConn()
  # And then connect the second to the first. 
  # Herein lies the "problem"
  i2.p = i1.p 

Currently this fails.
The rationale: NoConn as compared to an otherwise unconnected Signal is really just a marker, saying "I intend for this to connect nowhere". Hdl21's port-to-port connection semantics can then override this intent.

I think this should continue to fail.