Adds split_once to Source #678

Job	Run time
cargo-publish	0s
create-git-tag	0s
Tests (macos-latest, stable)	1m 15s
Tests (macos-latest, beta)	1m 21s
Tests (macos-latest, nightly)	1m 48s
Tests (windows-latest, stable)	2m 37s
Tests (windows-latest, beta)	2m 12s
Tests (windows-latest, nightly)	2m 24s
Tests (ubuntu-latest, stable)	34s
Tests (ubuntu-latest, beta)	1m 27s
Tests (ubuntu-latest, nightly)	1m 24s
	15m 2s

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