Make sure cirq gates and operations decompose to XPow/YPow/ZPow/CZPow + Measurement

[tanujkhattar]

Fixes #4858

This PR makes sure that most cirq gates and operations decompose to a default cirq gateset, which is defined to be XPow/YPow/ZPow/CZPow/Measurement and GlobalPhaseGate.

cirq.testing.assert_implements_consistent_protocols is extended to also check that if an object (gate / operation) has a _decompose_ method defined, then it should always end up decomposing to the cirq default gateset.

Note that some gates, like MatrixGates on more than 3 qubits, Controlled gates acting on qudits / multi-qubit matrix gates etc. can still not be decomposed into the default gateset always.

[MichaelBroughton]

Can we split this up into two PRs ? one for Matrix and one for Controlled ?

[tanujkhattar]

This cannot be split into two parallel PRs because I'm adding the new assert_decompose_ends_at_default_gateset to the standard assert_implements_consistent_protocols, which in-turn is then used in tests for all gates to make sure that their decomposition ends at the target gateset.

I can split it into 3 sequential PRs -- one that adds decomposition for Matrix gates, one for Controlled Gates and third which adds assert_decompose_ends_at_default_gateset, but I'll need to add redundant tests in the first 2 PRs for coverage, which will then be replaced by the standard assert_implements_consistent_protocols in the 3rd PR.

Let me know your preference.

[MichaelBroughton]

Hmmm could we do three just to be on the safe side ?

[tanujkhattar]

sure, I'll send the individual PRs shortly.