skiplist: add compare_insert #2356

Job	Run time
features (1.38)	1m 17s
features (nightly)	42s
test (1.38, ubuntu-latest)	5m 32s
test (1.38, windows-latest)	24m 33s
test (stable, ubuntu-latest)	9m 31s
test (stable, windows-latest)	20m 35s
test (nightly, ubuntu-latest)	3m 36s
test (nightly, windows-latest)	17m 1s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	10m 47s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	20m 53s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	24m 43s
clippy	37s
codegen	45s
careful	6m 17s
dependencies	23s
miri	18m 20s
rustfmt	19s
docs	16s
loom	2m 2s
san	6m 21s
shellcheck	11s
ci	0s
	2h 54m 41s

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