Refactor split_at/split_to #536

Job	Run time
miri	7m 59s
loom	2m 39s
cross (i686-unknown-linux-gnu)	29s
minrust	2m 31s
cross (armv7-unknown-linux-gnueabihf)	29s
nightly	3m 26s
cross (powerpc-unknown-linux-gnu)	53s
rustfmt	7s
cross (powerpc64-unknown-linux-gnu)	31s
stable (ubuntu-latest)	1m 22s
tsan	2m 48s
cross (wasm32-unknown-unknown)	8s
stable (macos-latest)	2m 22s
stable (windows-latest)	2m 4s
Publish Documentation	2m 10s
	29m 58s

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