feat: implement `|&` for function declarations #627

Job	Run time
Benchmarks	7m 24s
Source code checks (stable)	37s
Test (linux)	1m 57s
Source code checks (1.75.0)	24s
Build (x86_64/linux)	1m 7s
Test (macOS)	1m 54s
Build (aarch64/macos)	1m 22s
Build (aarch64/linux)	1m 38s
Build (wasm32/wasi-0.1)	56s
Build (x86_64/windows)	1m 35s
OS target tests (Fedora/latest)	25s
OS target tests (Debian/latest)	22s
OS target tests (Arch Linux/latest)	18s
	19m 59s

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