Make `DynamicMixer` and `OutputStream` generic #250

Job	Run time
cargo-publish	0s
Tests (macos-latest, stable)	1m 0s
Tests (macos-latest, beta)	1m 3s
Tests (macos-latest, nightly)	54s
Tests (windows-latest, stable)	1m 25s
Tests (windows-latest, beta)	1m 2s
Tests (windows-latest, nightly)	56s
Tests (ubuntu-latest, stable)	28s
Tests (ubuntu-latest, beta)	32s
Tests (ubuntu-latest, nightly)	34s
	7m 54s

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