Make CircuitData inner data accessible in Rust (Qiskit#12766)

* Add: `iter()` method to `CircuitData`
- Make `PackedInstruction` public.

* Docs: Remove "rust-native method" from docstring.

crates/circuit/src/circuit_data.rs

@@ -1167,4 +1167,9 @@ impl CircuitData {
             py_op: RefCell::new(inst.py_op.clone()),
         })
     }
+
+    /// Returns an iterator over all the instructions present in the circuit.
+    pub fn iter(&self) -> impl Iterator<Item = &PackedInstruction> {
+        self.data.iter()
+    }
 }

crates/circuit/src/circuit_instruction.rs

@@ -44,7 +44,7 @@ pub struct ExtraInstructionAttributes {
 
 /// Private type used to store instructions with interned arg lists.
 #[derive(Clone, Debug)]
-pub(crate) struct PackedInstruction {
+pub struct PackedInstruction {
     /// The Python-side operation instance.
     pub op: OperationType,
     /// The index under which the interner has stored `qubits`.