Adds cirq.num_cnots_required and cirq.to_special

[balopat]

Adds cirq.num_two_qubit_gates_required and cirq.to_special.

• cirq.num_cnots_required: Based on simple linear algebra users can calculate the minimum required two-qubit gates (CZ, CNOT) to implement a two-qubit unitary.
• cirq.to_special: converts a unitary to a special unitary

Context: this is the first PR breaking up #2873.

For an overview of the high level design decisions of the whole project see: https://drive.google.com/open?id=1SDEtttIAaTwfV9AUs7XAxeW3AUd0qLoY.

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[balopat]

Argh, I pushed this from my personal computer, I'll fix the CLA soon.

[googlebot]

CLAs look good, thanks!

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[CirqBot]

Automerge cancelled: No approved review.

[balopat]

note - I'm just testing automerge

[viathor]

Why is this in contrib and not in cirq.optimizers? The original bug suggests this is going to be very useful.

[balopat]

Why is this in contrib and not in cirq.optimizers? The original bug suggests this is going to be very useful.

@viathor, thank you for the thorough review - awesome suggestions!
I moved most of these functions into cirq packages, also updated the PR description to cater for them and fixed most of your comments. Let me know what you think!

[viathor]

Nice! LGTM with a few minor comments.

[CirqBot]

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