Chain extension

[pmikolajczyk41]

This PR includes:

1. pallet_contracts::ChainExtension which exposes a single function for calling pallet_snarcos::bare_store_key with mocked arguments
2. ink::chain_extension definition (reusable in any contract)
3. sample ink! contract that uses the extension

Reading:

• https://github.com/Cardinal-Cryptography/substrate/blob/aleph-v0.9.26/frame/contracts/src/chain_extension.rs
• https://github.com/paritytech/ink/tree/master/examples/rand-extension
• https://use.ink/macros-attributes/chain-extension/

[h4nsu]

One more thing, confused the hell out of me

[h4nsu]

I find it kinda weird this crate is under runtime folder. contracts would be much better place in my opinion

[deuszx]

I have to plead guilty here as I made @pmikolajczyk41 move it from /contracts to here.

My reasoning was (and still is) that the chain extension is part of the core chain (you can see it being defined in runtime/src/lib.rs ) and not living in the client space like contracts. Now it lives closer to the place where it's actually used and defined - we wouldn't be adding a chain extension written by clients (/contracts, even though now live in the node's repo, are not part of the core chain).

IMO this also lowers a congitive load on developers that will read the code. Since the chain extensions' functions are referred by an index (number) and not interface, having the caller's function index "closer" (in terms of code organization) to the actual index we will pattern match on should be it simpler to discover. For this particular case, consider the interface exposed to contracts (link) and the actual pattern matching in the chain extension that routes the calls (link).

We can still make it "publishable" and have contracts refer to it in their dependencies https://github.com/Cardinal-Cryptography/aleph-node/blob/liminal/chain-extension/contracts/snarcos-contract/Cargo.toml#L26 - making it all type-safe.

[pmikolajczyk41]

I see both points and I really cannot decide. There is, lets say, some solution with using https://github.com/Supercolony-net/obce, but for now it would be an overkill.

I'll leave this as it is now, and in case we find it inconvenient, we'll move it somewhere else (I suppose/hope somebody will play with it more and thus have some experience-based feedback).

CCing @Yithis to make him aware of these doubts (since he'll be extending this extension (today?))