Proposal: move GraphDevice out of contrib

[bryano]

See RFC. Questions and comments welcome.

@Strilanc @dabacon @viathor @maffoo @mrwojtek

[balopat]

It seems that we are a bit stalling with this one, I would like to drive this to a decision.

The maintainer team is a bit split: It seems that the current proposal is not ready to be accepted due to concerns about too much generality (@Strilanc) and overhead (@mrwojtek). There are some concerns around naming as well, specifically the crosstalk_graph (not_simultaneity_graph?). However, in #2185 it is clear that both @dabacon and @maffoo were advocating for storing adjacency information in Device.

Also there was not much activity from @bryano since May. @bryano - do you have the bandwidth to push this to completion and implement the feedback?

[balopat]

Discussed on Cirq Cynque:

• a graph based representation is definitely useful and most needed
• however, we need a bit less generic datastructure (no need to define gate per edge for example)
• next step: next version of the RFC needs to be approved