Identity gate

Identity is a unitary. We probably want an explicit representation for it in cirq.

Rationale:

1. Many expressions evaluate to the identity exactly (e.g. composition of H with H) or up to global phase (e.g. composition of X, Y and Z). Code that detects and optimizes such expressions may need to return identity explicitly.
2. Identity is a convenient initializer for reduce-like operations on gates.
3. It’s directly useful in expressions for quantum channels (see e.g. #1037).
4. It’s one of the basis elements in the Pauli basis of the space of hermitian operators. Having explicit representation for it simplifies code for #491.
5. There are currently multiple hacky ways of specifying the identity (e.g. cirq.Y**0). If an explicit, elegant way (e.g. cirq.I or cirq.Identity) is provided, the users are more likely to eschew the hacky expressions.

Some things to consider in implementation:

1. It should be removed by all decompositions.
2. It should compare equal to expressions such as cirq.Z**0 or cirq.Rx(rads=0).
3. Do we need a gate for changing global phase? If so, that gate should probably be implemented simply as a phase factor for identity.
4. Circuits such as

0: ───X^0───

1: ───Y─────

should display as

0: ─────────

1: ───Y─────

or

1: ───Y───

[dabacon]

I think displaying it as empty isn’t quite correct? Putting identity gates into a Moment is sort of saying "do nothing here and block anything else from happening here). If you don’t show it then you will end up having odd things where you don’t see that the identity gate is in a particular location.

[Strilanc]

Already done