Hashed provable type

[mitschabaude]

bindings: o1-labs/o1js-bindings#238

This follows up on #1376 with a variant of the same idea: Instead of representing a value by its packed field elements, we now represent it by its hash.

The benefit is that a hash is (potentially) even smaller than packed field elements -- it's a single field element. So, this can achieve a better improvement to constraint count as Packed, in the same situations.

The downside is that the overhead of hashing is much higher than that of packing, so there might be scenarios where either technique is better.

We use Hashed instead of Packed in ECDSA, which achieves a tiny improvement in combination with changing one of the default parameters for point caching (it's slightly worse with the original parameters, which were the optimal ones before, so we see that there's a trade-off. Also, packing seems still better for constant points, but I don't think it's worth adding the complexity of using different representation depending on whether the point is constant)

This also directly exposes the low-level tool underneath - Poseidon.hashPacked<T>(), to improve the Poseidon.hash(value.toFields()) pattern found in many code bases

[Trivo25]

it's a hashed point now!
// hashed points to make array access more efficient