feat: Operation and constructor declarations in `hugr-model` #2403

Job	Run time
Check for changes	4s
tests (Rust stable, no features)	1m 7s
check	1m 7s
Build benchmarks 🏋️	5m 27s
tests (Rust nightly, coverage)	2m 22s
tests (Rust 1.75)	1m 59s
tests (Rust stable, all features)	1m 42s
tests (Rust beta)	1m 48s
rs-semver-checks / Rust semver-checks 🦀	3m 12s
Check standard extensions	4s
Required checks 🦀	1s
	18m 53s

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