Refactor split_at/split_to #562

Job	Run time
nightly	3m 57s
minrust	3m 16s
cross (i686-unknown-linux-gnu)	40s
miri	7m 59s
cross (armv7-unknown-linux-gnueabihf)	28s
loom	3m 0s
cross (powerpc-unknown-linux-gnu)	38s
rustfmt	6s
stable (ubuntu-latest)	1m 8s
cross (powerpc64-unknown-linux-gnu)	31s
stable (macos-latest)	1m 39s
tsan	3m 24s
cross (wasm32-unknown-unknown)	6s
stable (windows-latest)	3m 24s
Publish Documentation	2m 54s
	33m 10s

Provide feedback