Add FileInpustStream::available() #541

Job	Run time
rust_ci (macos-latest, beta)	1m 10s
rust_ci (macos-latest, stable)	40s
rust_ci (ubuntu-latest, stable)	52s
rust_ci (ubuntu-latest, nightly)	1m 29s
rust_ci (windows-latest, beta)	3m 4s
rust_ci (macos-latest, nightly)	1m 14s
rust_ci (ubuntu-latest, beta)	1m 32s
rust_ci (windows-latest, nightly)	2m 44s
rust_ci (windows-latest, stable)	3m 5s
	15m 50s

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