Refactor: Allow manifest data types to serialize and deserialize themselves

[awvwgk]

Like the dependency tree all package manifest types should be able to deserialize and serialize themselves.

This is likely to conflict with any changes at the manifest data types and therefore deferred until we decided how to proceed with #498.

Currently, our target data structure would be the representation provided by TOML-Fortran, but in the future we might use a more general data type provided by stdlib (see fortran-lang/stdlib#479).