refactor: Put all available methods under `method.*` (#23)

BREAKING CHANGE:

- Instead of importing methods (like `bond`, `balanceTransfer`) directly from the root, the package exposes a `methods` object in the root, and this object contains all available methods organized by pallet:

```diff
- import { bond, balanceTransfer } from '@amaurymartiny/txwrapper';
+ import { methods } from '@amaurymartiny/txwrapper';

- bond(...);
- balanceTransfer(...);
+ methods.staking.bond(...);
+ methods.balances.tranfer({dest: '...', value: }, {blockHash: '0x...'});
+ methods.balances.transferKeepAlive(...);
```

- All the methods under `methods.*.*` take now 2 arguments: the first one is the actual args specific to the tx method, and the second one is `BaseTxInfo`, common to all transactions

```typescript
methods.staking.nominate({target: ['F1...']}, {blockHash: '0x..', ...});
```

- `balances::transfer` arguments has been renamed from `{to, amount}` to `{dest, value}`, to reflect the rust codebase:

```typescript
methods.balances.transfer({dest: '5....', value: 100}, {blockHash: '0x..', ...});
```