Serialized type for the packman library, instances and helpers

The data type Serialized a includes a phantom type a to ensure type safety within one and the same program run. Type a can be polymorphic (at compile time, that is) when Serialized a is not used apart from being argument to deserialize.

The Show, Read, and Binary instances of Serialized a require an additional Typeable context (which requires a to be monomorphic) in order to implement dynamic type checks when parsing and deserialising data from external sources.

The power of evaluation-orthogonal serialisation is that one can externalise partially evaluated data (containing thunks), for instance write it to disk or send it over a network. Therefore, the module defines a Binary instance for 'Serialized a', as well as instances for Read and Show@ which satisfy

read . show == id :: 'Serialized' a -> 'Serialized' a

The phantom type is enough to ensure type-correctness when serialised data remain in one single program run. However, when data from previous runs are read from an external source, their type needs to be checked at runtime. Type information must be stored together with the (binary) serialisation data.

The serialised data contain pointers to static data in the generating program (top-level functions and constants) and very likely to additional library code. Therefore, the exact same binary must be used when reading in serialised data from an external source. A hash of the executable is included in the representation to ensure this.