Use Arc instead of Box for internal expression nodes in SymbolExpr (Qiskit#14660)

This commit updates the internal construction of the SymbolExpr struct
use a reference counting pointer instead of a regular pointer to a heap
allocated object. The way the binary tree gets constructed internally
using a normal pointer results in a lot of a copies of elements on the
tree. This creates a large performance overhead for the symbol expr as
we end up copying and freeing elements on the binary tree a large
amount. This was the root cause of the regression reported in Qiskit#14653
as the expression that gets generated by the transpiler internally ends
up adding a lot of elements to global phase expression and that results
in a lot of memory allocation and deallocation overhead. By using a
reference counting pointer instead of creating and freeing copies of
the expressions on the tree we instead only keep a single copy and
just increment or decrement the reference count.

Fixes Qiskit#14653

Author: mtreinish

crates/circuit/src/parameter_expression.rs

@@ -21,6 +21,7 @@ use num_complex::Complex64;
 use std::collections::hash_map::DefaultHasher;
 use std::fmt;
 use std::hash::{Hash, Hasher};
+use std::sync::Arc;
 
 use pyo3::prelude::*;
 use pyo3::IntoPyObjectExt;
@@ -84,7 +85,7 @@ impl ParameterExpression {
             .replace("__end_sympy_replace__", "$");
 
         ParameterExpression {
-            expr: SymbolExpr::Symbol(Box::new(name)),
+            expr: SymbolExpr::Symbol(Arc::new(name)),
         }
     }