Encode how to map provables to normal values

[mitschabaude]

bindings: o1-labs/o1js-bindings#213

this adds a new generic type argument and two new functions to Provable<T>:

type Provable<T, V> = {
  // ...
  toValue(t: T): V;
  fromValue(v: V): T;
}

the idea is that we can automatically convert between provable types and their normal JS equivalents, and stop leaking provable types outside actual provable code.

Motivation:

• we can make many interfaces more ergonomic by accepting JS values, in an automated way
• we hope to reduce confusion about provable constants, because people hopefully will never have to use them (they just use JS values), so the constant version of provable types become an internal detail

Some notes:

• There is a new From<Provable<T>> type, e.g. From<typeof Field> = Field | bigint | string | number, which is automatically derived from deeply inspecting provable types and their fromValue() methods. The DX of using it is surprisingly nice -- the type is fully resolved in Intellisense

• The PR uses fromValue for making the return value of the witness() callback accept JS values. This is super nice IMO and hints at what we could do with this going forward. For example, we could have a wrapper for all gadgets / provable methods which takes both a provable implementation (as before) and a constant implementation which receives js values, and the result takes the flexible From<> type

• To make the example above work, I reimplemented CircuitString with Struct and a JS type of string. It also works with CircuitValue but I wanted to have an example for how to override the JS value type for Structs, it's a bit more involved since Struct wants to infer everything for you (while CircuitValue is somewhat weakly typed)

• For backwards compatibility, the V type parameter is optional in the exported Provable, so you can still write Provable<T> in situations where the JS value doesn't matter

[MartinMinkov]

Can you make an issue and reference it to the comment to tackle later? (if you don't mind, of course)

I'd like to see more issues in TODO comments like this to make it easier to pick up :D

[MartinMinkov]

If you think it's a better idea, can you put 6 into https://github.com/o1-labs/o1js/blob/main/src/lib/mina/constants.ts? It's a minor change, but it could really help explain later why it's this value.

[mitschabaude]

TODO: forgot to add changelog for this