Serializable provable types

[mitschabaude]

For some applications (see zksecurity/mina-credentials#18) it would be nice if o1js provable types could support a serialized form.

It's fine if this is possible for just a subset of provable types, like a list of base types and common combinators like Struct.

Serializable types would, for example, enable us to specify the logic for certain simple ZkPrograms without access to o1js directly, for example, inside a browser extension or mobile app.