Add parameters for PauliString and GlobalPhase

[daxfohl]

Allows parameters for coefficient of PauliString and GlobalPhase:

• Fix up the types.
• Only cast to complex in initializer if it's numeric.
• Add handlers for parameter resolution.
• Raise errors in a couple functions that do non-basic math on coefficient (most functions just multiply, which should be fine).
• Return False from has_unitary. (Side note, should we just add that check to the protocol method itself--we repeat it everywhere?)
• Add a block for circuit_diagram_info when the coefficient is parameterized.
• Add tests, update pydocs.

Also tests and fixes #4508 @tanujkhattar

[tanujkhattar]

@daxfohl Great timing! I added a comment / complaint in #5085 today that GlobalPhaseGate don't support symbols. Looks like they will soon. :))

[daxfohl]

(Note that contrary to what I said the linked issue, decompose doesn't work when parameterized anyway since it's not unitary, so we actually didn't need to change GlobalPhaseGate here. But I'd already done it before I noticed it was unnecessary, so I figured I may as well leave the change in place since it seems non-invasive and probably useful.)

[daxfohl]

Fixed diagonalgate while I was here. Required changing np.exp(1j * hat_angles[0])) to 1j ** (2 * hat_angles[0] / np.pi) because np.exp breaks on a symbol.

[daxfohl]

@tanujkhattar do you mind taking this one?

[tanujkhattar]

LGTM % nits.

This is great, Thanks!

[tanujkhattar]

LGTM! Thanks a lot for the changes!

[CirqBot]

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[daxfohl]

@tanujkhattar Can you kick cirqbot again?