Support flexible circuit values

[mitschabaude]

• companion to https://github.com/o1-labs/snarkyjs/pull/416
• changes methods on primitive types: ofFields -> fromFields, ofBits -> fromBits, to be compatible with the new generalized typ / CircuitValue in JS
• expose a more low-level variant of exists to JS, and define the public-facing Circuit.witness on the JS side instead, so that we can manage our CircuitValues in JS (the OCaml side will just witness an array of field elements)