Add ByteArrayOutputStream::size #556

Job	Run time
rust_ci (macos-latest, stable)	52s
rust_ci (macos-latest, nightly)	1m 32s
rust_ci (ubuntu-latest, nightly)	1m 37s
rust_ci (ubuntu-latest, stable)	1m 2s
rust_ci (macos-latest, beta)	52s
rust_ci (ubuntu-latest, beta)	1m 4s
rust_ci (windows-latest, nightly)	2m 45s
rust_ci (windows-latest, beta)	1m 52s
rust_ci (windows-latest, stable)	2m 24s
	14m 0s

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