Refactor split_at/split_to #537

Job	Run time
loom	2m 36s
minrust	2m 32s
miri	8m 0s
cross (i686-unknown-linux-gnu)	29s
nightly	3m 29s
cross (armv7-unknown-linux-gnueabihf)	28s
rustfmt	5s
stable (ubuntu-latest)	1m 16s
cross (powerpc-unknown-linux-gnu)	32s
stable (macos-latest)	2m 21s
cross (powerpc64-unknown-linux-gnu)	31s
tsan	2m 51s
stable (windows-latest)	2m 57s
cross (wasm32-unknown-unknown)	38s
Publish Documentation	2m 12s
	30m 57s

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