Introduce a public constant for Rodio's default sample rate #547

Job	Run time
Tests (macos-latest, stable)	56s
cargo-publish	0s
create-git-tag	0s
Tests (macos-latest, beta)	51s
Tests (macos-latest, nightly)	54s
Tests (windows-latest, stable)	1m 44s
Tests (windows-latest, beta)	1m 53s
Tests (windows-latest, nightly)	1m 53s
Tests (ubuntu-latest, stable)	1m 13s
Tests (ubuntu-latest, beta)	1m 9s
Tests (ubuntu-latest, nightly)	1m 4s
	11m 37s

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