Make `DynamicMixer` and `OutputStream` generic #251

Job	Run time
cargo-publish	0s
Tests (macos-latest, stable)	55s
Tests (macos-latest, beta)	49s
Tests (macos-latest, nightly)	56s
Tests (windows-latest, stable)	1m 25s
Tests (windows-latest, beta)	1m 3s
Tests (windows-latest, nightly)	55s
Tests (ubuntu-latest, stable)	58s
Tests (ubuntu-latest, beta)	30s
Tests (ubuntu-latest, nightly)	1m 4s
	8m 35s

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