add generateMerkleProof and generateMerkleUpdate functions for dictionaries

[Gusarich]

Easiest way to use merkle proofs / updates from FunC is through dictionaries. I thought it will be nice to have some functionality to generate those exotic cells from this library.

I've tested these methods on some FunC code and it worked correctly. I've also added some tests that utilise exoticMerkleProof functions which check correctness too.

Here's an example on how to use it.
TS:

let d = Dictionary.empty(Dictionary.Keys.Uint(8), Dictionary.Values.Uint(32));
d.set(1, 11);
d.set(2, 22);
d.set(3, 33);

// we want to prove that there is a key `3` with corresponding value `33` in the dictionary to a smart contract that only stores the hash of the whole dictionary
const proof: Cell = d.generateMerkleProof(3);

FunC:

(slice, int) begin_parse_exotic (cell c) asm "XCTOS";

int check_proof (int merkle_root, cell merkle_proof, int index, slice expected_data) {
    (slice cs, int special?) = merkle_proof.begin_parse_exotic();

    if (~ special?) {
        return false;
    }
    if (cs~load_uint(8) != 3) { ;; Merkle proof
        return false;
    }
    if (cs~load_uint(256) != merkle_root) {
        return false;
    }

    cell dict = cs~load_ref();
    (slice data, int found?) = dict.udict_get?(8, index);

    if (~ found?) {
        return false;
    }

    return equal_slices(data, expected_data);
}

[Gusarich]

I opened this PR to ton-core org at first, but looks like it was moved here.
ton-core#29

[Gusarich]

I just found a bug in the merkle proof generation. Going to fix it and optimise as well.

[Gusarich]

@dvlkv I fixed the generation of Merkle proofs! Also made the code more straight-forward.

I have also additionally tested it in my Airdrop project and it works as intended:

• https://github.com/Gusarich/airdrop