Add FileDescriptorId abstraction for serializable file descriptors #670

Job	Run time
rust_ci (windows-latest, stable)	50s
rust_ci (ubuntu-latest, stable)	38s
rust_ci (macos-latest, stable)	41s
rust_ci (ubuntu-latest, beta)	39s
rust_ci (macos-latest, beta)	39s
rust_ci (windows-latest, beta)	50s
	4m 17s

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