Make `DynamicMixer` and `OutputStream` generic #252

Job	Run time
cargo-publish	0s
cargo-publish	0s
Tests (macos-latest, stable)	49s
Tests (macos-latest, stable)	1m 8s
Tests (macos-latest, beta)	54s
Tests (macos-latest, beta)	54s
Tests (macos-latest, nightly)	1m 13s
Tests (macos-latest, nightly)	51s
Tests (windows-latest, stable)	2m 3s
Tests (windows-latest, stable)	1m 10s
Tests (windows-latest, beta)	1m 9s
Tests (windows-latest, beta)	54s
Tests (windows-latest, nightly)	1m 2s
Tests (windows-latest, nightly)	58s
Tests (ubuntu-latest, stable)	13s
Tests (ubuntu-latest, stable)	19s
Tests (ubuntu-latest, beta)	33s
Tests (ubuntu-latest, beta)	27s
Tests (ubuntu-latest, nightly)	27s
Tests (ubuntu-latest, nightly)	26s
	15m 30s

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