Schema definition and fully dynamic subschemas

[mitschabaude]

We should create a well-defined way to define schemas, that feels general and not o1js-specific.

Support general data types:

• strings (=> DynamicString), numbers (=> UInt64 or Field?), booleans (=> Bool)
• records, (dynamic length) arrays

The above can be inferred from any JSON value!

Also, we'll need extra data types that need and annotated schema to be understood

• Mina-specific types like Field, PublicKey
• (add later) maybe fixed-length Arrays, Bytes, Strings
• (add later) maybe foreign fields? whatever turns out to be useful later

Later, when creating the circuit, we should only need the rough form of the original schema that some data adhers to.

In particular:

• need to pick a max size for strings/arrays, but don't have to know any (max) size that was picked for the original schema -- just that it's a string
• only need partial schemas for records, and that should also work for nested records!