Add representation of "switch"

[jakelishman]

Summary

This simply adds the representation of a switch statement to Terra, without any of the extra supported needed anywhere else in the package. These will come in follow-up commits.

Details and comments

It's possible there are references to a builder interface left in this PR in places - I tried to squash all of those when I rebased my original branch (I'll add the builder in a follow-up, and it still needs tests writing). I can either fix them in this PR, or just leave it to make sense when the builder interface is added again.

There's no support added in the rest of the library for this new operation yet; I'll do that in follow-ups.

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[coveralls]

Pull Request Test Coverage Report for Build 4565467629

• 64 of 76 (84.21%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 85.379%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/circuit/controlflow/switch_case.py	59	71	83.1%

Totals
Change from base Build 4564463001:	-0.001%
Covered Lines:	67319
Relevant Lines:	78847

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💛 - Coveralls

[kevinhartman]

LGTM overall.

I'm digging this interface—very clean!

[kevinhartman]

LGTM!

[jakelishman]

Let me poke @kdk for review before as well, since most of the control-flow representation stuff is his.

[kdk]

LGTM

[jakelishman]

From offline Slack discussions, and just for posterity (it's me typing):

for params: I put the circuit bodies in the params so that QuantumCircuit.assign_parameters could continue to work in its current form (though ideally I’d have that use ControlFlowOp.blocks), but I didn’t use the case labels there because there’s really nothing meaningful you could possibly do with them via params

any attempt to modify them would likely invalidate the data structures, and if they’re mashed into a flat structure in params, they’d be no use to the user