Take port-references to `Pair`, and any other `InstanceBundle`

[dan-fritchman]

@h.module 
class M:
  inp = h.Diff()
  out = h.Diff()
  bias, VSS = 2 * h.Input()

  pair = h.Pair(h.Nmos())(d=out, g=inp, b=VSS)
  nbias = h.Nmos()(g=bias, s=VSS, b=VSS, d=FIXME) # <= FIXME here the problem

It would be nice if this could say

  nbias = h.Nmos()(g=bias, s=VSS, b=VSS, d=pair.s) # <= Connect into a port-reference of the `Pair`

But that doesn't quite work yet.