feat: Operation and constructor declarations in `hugr-model` #2387

Job	Run time
Check for changes	3s
Build benchmarks 🏋️	6m 16s
tests (Rust 1.75)	1m 58s
check	1m 6s
tests (Rust nightly, coverage)	2m 9s
tests (Rust beta)	1m 49s
tests (Rust stable, all features)	1m 34s
tests (Rust stable, no features)	1m 18s
rs-semver-checks / Rust semver-checks 🦀	3m 25s
Check standard extensions	5s
Required checks 🦀	1s
	19m 44s

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