Reintroduced diagnostic::on_unimplemented & rustc 1.78 dependency #288

Job	Run time
Contracts (nightly) / Rust tests	3m 21s
Contracts (nightly) / Clippy linter check	1m 28s
Contracts (nightly) / Test Coverage	6m 57s
Contracts (nightly) / Wasm tests	10m 36s
	22m 22s

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