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0b11c28001
The specification defines an algorithm to calculate a `ChainID`, which can be used to identify the result of subsequent applications of layers. Because this algorithm is subtle and only needs to implemented in a single place, we provide a reference implementation. For convenience, we provide functions that calculate all the chain ids and just the top-level one. It is is integrated with the distribution/digest type for safety and convenience. As part of this, the `identity` package has been introduced. For consuming code, a few helpers have been provide to ease transition as the name of the upstream package has not yet been finalized. Users of this package should employ `FromBytes`, `FromString` and `FromReader` where appropriate, which should ease the transition if these packages change. Tests are formulated based on pre-calculation of chain identifiers to ensure correctness. Signed-off-by: Stephen J Day <stephen.day@docker.com>