Implement `$struct$unnest()` #672

Job	Run time
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 5s
build (ubuntu-20.04, aarch64-unknown-linux-gnu)	15m 14s
build (macos-14, x86_64-apple-darwin)	17m 14s
build (macos-14, aarch64-apple-darwin)	16m 58s
build (windows-latest, x86_64-pc-windows-gnu)	30m 42s
test (macos-12, release)	5m 7s
test (macos-14, release)	2m 1s
test (windows-latest, oldrel-1)	5m 47s
test (windows-latest, release)	10m 44s
test (windows-latest, devel)	4m 37s
test (ubuntu-latest, oldrel-1)	3m 3s
test (ubuntu-latest, release)	2m 18s
test (ubuntu-latest, devel)	7m 47s
merge	8s
release	0s
	2h 15m 45s

Provide feedback