Add QkDag instruction appliers and getter to C API

[jakelishman]

This adds qk_dag_get_instruction as a mirror to
qk_circuit_get_instruction (unifying the code in anticipation of making it lower-allocation in the future), and the standard-instruction application functions qk_dag_apply_measure, qk_dag_apply_barrier and qk_dag_apply_reset.

We're still missing qk_dag_apply_unitary, but that's for a different commit.

Summary

Details and comments

Close #15193
Close #15311

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 19687548632

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 111 of 134 (82.84%) changed or added relevant lines in 2 files are covered.
• 245 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.01%) to 88.243%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/cext/src/circuit.rs	31	32	96.88%
crates/cext/src/dag.rs	80	102	78.43%

Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	82.3%
crates/circuit/src/parameter/symbol_expr.rs	1	73.35%
crates/qasm2/src/expr.rs	1	93.82%
crates/transpiler/src/passes/unitary_synthesis.rs	1	92.57%
crates/cext/src/dag.rs	3	84.04%
crates/qasm2/src/lex.rs	4	92.29%
crates/circuit/src/dag_circuit.rs	234	84.53%

Totals
Change from base Build 19678997564:	0.01%
Covered Lines:	94483
Relevant Lines:	107071

---

💛 - Coveralls

[Shobhit21287]

Hi Jake, this looks good! Until we can represent operations generally, its good to have these as different function.

[jakelishman]

Shobit: thanks for doing the review! When you're submitted a PR review, there's a choice of three options for the review: "comment", "approve" and "request changes". If your review is saying "I think this is good to merge", you want to check the "approve" button instead of leaving it on "comment".

(We don't usually use "request changes" - we just leave those as "comment" - because "request changes" puts an actual block on merge, and requires the same reviewer to come back and dismiss their stale review.)

[raynelfss]

This functionally looks good to me, I saw a couple of things here and there when it came to documentation. And, assuming because of #15221 we won't be adding a release note here. So feel free to ping me once the documentation issues are solved.

[raynelfss]

Is this meant to be nul or is it a typo? It shows up later in the code so I'm assuming it's some meaning I don't know of, but in case it isn't.

Suggested change

	/// Panics if the operation name contains a nul, or if the instruction has non-float parameters.
	/// Panics if the operation name contains a null, or if the instruction has non-float parameters.

[jakelishman]

The ASCII \0 byte is referred to by both "nul" and "null" - the former comes from its three-letter short representation. We use nul elsewhere in the docs for it at the moment, so we might want to tidy up uniformly later.

[raynelfss]

This looks good to me. There's a couple things in the documentation that could be improved upon though.