feat: r/demo/foo20airdrop - Merkle Airdrop contract example

[albttx]

Description

This PR bring an example for a merkle airdrop contract.

• the package p/demo/airdrop
• the realm r/demo/foo20airdrop

How it's works ?

A merkle airdrop contract works by saving off-chain the elements of the merkle tree. ONLY the Merkle root is saved on the realm.

To confirm an element is present, we just need to send to the contract a merkle proof.

[albttx]

waiting for: #893 and #895 to undraft

[moul]

For better control and data integrity, keep the fields in the Airdrop struct unexported.

To maintain easy marshaling with the Bytes helper, consider this approach:

type Airdrop struct {
    data struct {
        Address std.Address
        Amount  uint64
    }
}

[zivkovicmilos]

Looks good 💯

Please see @moul's comments, otherwise we should be good to go

[moul]

Just blocking since there are several changes. Let’s make an additional round of reviews.

[albttx]

There is tests issue once rebased with master.

There is a situation to figure out, with grc20.IGRC20, because it return a grc20.UserToken .

When user1.gno -> r/foo20-airdrop get foo20.GRC20() and then from r/foo20-airdrop exec foo20.GRC20().Transfert, std.PrevRealm isn't the realm but the user ...

Possible fix could be to update in user_token.gno to use CurrentRealm rather than PrevRealm

[moul]

Related (and depending?) on #952

[zivkovicmilos]

Hey @albttx,

What's the status of this PR?

[albttx]

(sorry force-pushed, missed the --no-rebase )