feat: function type #690

Job	Run time
clippy	1m 32s
docs	52s
feature checks	2m 22s
fmt	18s
check WASM	35s
test stable --no-default-features	1m 16s
test stable	51s
test stable --all-features	1m 25s
test beta --no-default-features	53s
test beta	55s
test beta --all-features	1m 4s
test nightly --no-default-features	2m 12s
test nightly	1m 16s
test nightly --all-features	1m 39s
test 1.65 --no-default-features	1m 45s
test 1.65	1m 32s
test 1.65 --all-features	1m 45s
	22m 12s

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