Circuit._repr_svg_ with nice diagrams for ipython

[mpharrigan]

https://ipython.readthedocs.io/en/stable/config/integrating.html#rich-display

Also see https://github.com/perak/quantum-circuit for a MIT licensed package that makes nice SVG circuits

[mpharrigan]

Partially done in #2323

I'm not going to work on it any more. Someone could pick this up and address the following things

• make it prettier
• actually distribute the horizontal spacing / don't assume constant row-height
• thorough testing
• I think it breaks if you have empty moments (?)
• Generalize the textdiagramdrawer / blockdiagramdrawer to reduce duplicated code
• Move from contrib to main trunk
• Add _repr_svg_ to cirq.Circuit instead of requiring wrapping in a helper class (this one must come after the others)

[mpharrigan]

• fix spacing routines to not require integer coordinates

[mpharrigan]

• deal with MatrixGate