[sui-framework] Adds Balance module

[damirka]

Covers #1233.

This PR has a draft status and used for demonstration purposes.
UPD: converted to normal PR as it seems complete.

Pros

1. Balance is a small and nice module with zero dependencies;
2. It is possible to implement custom wrappers on a Balance type;
3. Coin is transferable wrapper around Balance; possibly not needed in some applications;
4. Separation of modules resolves name collisions and helps readability (Coin is for minting and handling accounts, balance is its internals);
5. Balance doesn't use TxContext at all and has no IDs - which is great!

Cons

1. Multi-layer solutions complicate things;
2. We'll need to have a guide/tutorial on how to use them (def!); and when to use Coin or Balance;
3. Since Coin handles minting, Balance is always bound to it and cannot be used freely for custom implementations of finances;
4. To take a Coin from a balance, few calls needed:

let balance = Balance { value: 100 }; // roughly
let coin = Coin::from_balance(Balance::split(&mut balance, 50), ctx);

[tnowacki]

A few random comments:

1. "Since Coin handles minting, Balance is always bound to it and cannot be used freely for custom implementations of finances;". You could handle this with a minting capability. But then you will have like Balance<MintCap, CoinType> everywhere. Could be a nice usecase for type aliases though (Coin::Balance<CoinType> = Balance<Coin::MintCap, CoinType>)
2. For me it feels a bit strange to have Coin be the one with an ID, but maybe that is too much time in Diem... I could imagine something like (waves hands) Coin { value: u64 } and Wallet { id: VersionedId, balance: Coin } (not that the term "Wallet" is overused at all or anything)

[sblackshear]

This feels a bit asymmetric with Coin::zero to me. I don't have a strong preference for empty vs zero, but I think we should probably pick the same name for both.

[sblackshear]

Nit: comments on error codes and more descriptive names