Serializable and hashable program spec

[mitschabaude]

A program Spec currently relies on o1js datatypes. However, we will need to transfer the program spec to a different web page.
Furthermore, it would be desirable to specify a credential from plain text or from a browser interaction.

For both use cases, we need a way to serialize & deserialize the spec to/from a simple data representation like JSON or a string.

To achieve this, we might have to predefine a list of supported o1js base type and define a serialization for each of them.

Finally, once we have a serialization, add a way to hash that serialization to get a unique short string of bytes that represents a spec.
Careful: equivalent specs must be serialized to exactly the same output (i.e. order of keys in a record should be deterministic), otherwise hashes for them will be different.