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

[mergify]

Summary

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 #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.

Details and comments

Fixes #14653

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This is an automatic backport of pull request #14660 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 15864966871

Details

• 48 of 71 (67.61%) changed or added relevant lines in 3 files are covered.
• 35 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 87.969%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/circuit/src/symbol_parser.rs	3	4	75.0%
crates/circuit/src/symbol_expr.rs	44	66	66.67%

Files with Coverage Reduction	New Missed Lines	%
crates/transpiler/src/passes/unitary_synthesis.rs	1	94.57%
crates/circuit/src/symbol_expr.rs	4	73.85%
crates/qasm2/src/lex.rs	6	91.48%
crates/qasm2/src/parse.rs	24	96.22%

Totals
Change from base Build 15742171824:	-0.03%
Covered Lines:	83036
Relevant Lines:	94392

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💛 - Coveralls