Refactor split_at/split_to (#663) #566

Job	Run time
miri	28s
loom	3m 41s
cross (i686-unknown-linux-gnu)	29s
minrust	3m 22s
cross (armv7-unknown-linux-gnueabihf)	27s
cross (powerpc-unknown-linux-gnu)	29s
nightly	4m 30s
cross (powerpc64-unknown-linux-gnu)	28s
rustfmt	3s
cross (wasm32-unknown-unknown)	7s
stable (ubuntu-latest)	1m 9s
stable (macos-latest)	1m 37s
tsan	3m 56s
stable (windows-latest)	2m 32s
Publish Documentation	3m 28s
	26m 46s

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