Fix classical control diagram symbols and svg

[daxfohl]

Fixes #5688. Fixes #5689.

A quick image search for circuit diagrams shows that the convention seems to be arrows for measurement, which I represent here in diagrams with V. I followed the suggestion from #5688 to use the quantum control symbol @ to represent classical control as well.

For SVG, now the circuit from the teleportation example renders in a more standard fashion, with double-lines for classical registers and measurements, and arrows representing classical data flow:

(previously looked like this, which is obviously misleading):

I avoided the suggestion from #5688 to represent measurement as X in diagrams. The rationale is that X implies a bit flip. However in Cirq, multiple measurements to the same register will be stored independently in an array. So, representing those as X, which implies a bit flip, would be a misrepresentation.

I'll leave it open to discussion on whether this is correct and/or better. It's a three-character change in the source and then a global find-and-replace to patch up the tests and docs, so it's easy to accommodate whatever y'all decide here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.65%. Comparing base (02941bf) to head (eaa0367).
Report is 29 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7233   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files        1106     1106           
  Lines       95869    95882   +13     
=======================================
+ Hits        94581    94594   +13     
  Misses       1288     1288           

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[mhucka]

I tagged this to be discussed at the next Cirq Cynq.

A change like this strikes me as a breaking change. I see in the comments on #5688 this concerns was brought up a couple of years ago, but not resolved at the time.

[daxfohl]

Yeah I understand the concern. If it's decided that this is breaking, we should also probably revisit other issues involving presentation of circuit diagrams (e.g. #1245, #3597, #3612, #3913, etc), SVG (#5689), 3D view, Bloch sphere, and so on, and close anything else that would be considered breaking.