Implement `$struct$unnest()` #677

Job	Run time
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 23s
build (ubuntu-20.04, aarch64-unknown-linux-gnu)	15m 21s
build (macos-14, x86_64-apple-darwin)	15m 6s
build (macos-14, aarch64-apple-darwin)	17m 35s
build (windows-latest, x86_64-pc-windows-gnu)	31m 33s
test (macos-12, release)	6m 23s
test (macos-14, release)	1m 57s
test (windows-latest, oldrel-1)	5m 29s
test (windows-latest, release)	7m 57s
test (windows-latest, devel)	4m 20s
test (ubuntu-latest, oldrel-1)	2m 55s
test (ubuntu-latest, release)	2m 39s
test (ubuntu-latest, devel)	7m 45s
merge	13s
release	0s
	2h 13m 36s

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