Use singleton canonical instances for OpenQASM 3 gate lookups

[jakelishman]

Summary

This reduces most symbol-table lookups for gate calls to simple is lookups, which is a sizeable increase in performance for the majority of large circuits passed to the OpenQASM 3 exporter. These circuits are typically entirely dominated by standard-library gate calls.

Details and comments

This is a very simple tweak with a pretty large impact on performance compared to the 1.2 series for circuits compiled into standard-library ones, since it near completely erases the overhead of the gate-based symbol lookup.

[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 11580872225

Details

• 21 of 21 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.009%) to 88.677%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	1	93.48%
crates/accelerate/src/unitary_synthesis.rs	1	92.2%
<!--	Total:	2

Totals
Change from base Build 11579524174:	0.009%
Covered Lines:	75350
Relevant Lines:	84971

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💛 - Coveralls