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The command-line options to c2a are already long and will only get longer as new arguments are added. This allows it to load them from a config file. For simplicity of implementation, the config file supplements the command-line arguments and uses the same format they do, although it ideally will replace them over time. This also adds an option to dump the current set of command-line arguments to a config file and exit. This is intended to ease migration over to the config file: for a given crate, we can call c2a with its command-line arguments plus this new flag to generate the config file and then use just that instead. Test: Dump and load some config files. Change-Id: I63b29dd20bcff5d5832dbd380d7c6eb273547ed0