Lift `Sink::clear` into `SpatialSink` #253

Job	Run time
cargo-publish	0s
Tests (macos-latest, stable)	1m 4s
Tests (macos-latest, beta)	1m 11s
Tests (macos-latest, nightly)	1m 26s
Tests (windows-latest, stable)	1m 27s
Tests (windows-latest, beta)	1m 25s
Tests (windows-latest, nightly)	1m 15s
Tests (ubuntu-latest, stable)	33s
Tests (ubuntu-latest, beta)	35s
Tests (ubuntu-latest, nightly)	27s
	9m 23s

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