Emit twirl from boundary in left-dressed boxes

[joshuasn]

Summary

This PR moves the location of twirl emissions from the right-hand side of a left-dressed box to the boundary between easy and hard gates. Adding to allow for support of measure operations as propagators.

Details and comments

This simplifies the sorted order pretty significantly.

I also had to make a small change where the choice between SliceRegisterNode and CombineRegistersNode was made. Previously, when we had a left- then a right-dressed box, as the emits were on the edge of the box opposite the hard content, the emissions from both would be combined to always match the number of subsystems for any hard gates. This is not the case anymore for examples like the following:

with circuit.box([Twirl()]):
     circuit.cx(0, 1)

with circuit.box([Twirl()]):
     circuit.h(0)

with circuit.box([Twirl(dressing="right")]):
     circuit.cx(0, 1)

Here, the emission on qubit 1 from the right dressed box would error when encountering the CX from the first box as it selects a SliceRegisterNode that is trying to write a single register to two registers. The change in this PR looks for these cases and instead uses a CombineRegisterNode that starts with an identity on the correct number of subsystems and multiplies the virtual register on a smaller number of subsystems into it.

[ihincks]

Thanks Josh. We should include a changelog for the change here.