perf: Return `Cow<Signature>` where possible #2871

Job	Run time
Check for changes	5s
tests (Rust nightly, coverage)	4m 41s
Build benchmarks 🏋️	2m 39s
check	1m 29s
tests (Rust 1.75)	2m 56s
tests (Rust stable, all features)	2m 29s
tests (Rust beta)	3m 0s
tests hugr-llvm	0s
tests (Rust stable, no features)	1m 47s
Check standard extensions	4s
Required checks 🦀	1s
	19m 11s

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