Add FileDescriptorId abstraction for serializable file descriptors #669

Job	Run time
rust_ci (windows-latest, stable)	3m 5s
rust_ci (macos-latest, stable)	1m 42s
rust_ci (windows-latest, beta)	3m 3s
rust_ci (ubuntu-latest, beta)	1m 26s
rust_ci (macos-latest, beta)	1m 21s
rust_ci (ubuntu-latest, stable)	1m 25s
	12m 2s

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