Give names for intermediate structs

Consider following contract

```solidity
contract TypechainTest {
struct Input {
uint256 a;
int128 b;
bool c;
}

struct Output {
uint256 d;
int128 e;
}

function test(Input calldata input) public pure returns (Output memory result) {
result.d = input.a;
result.e = input.b;
}
}
```

it leads to following generation:

```ts
...
test(
input: { a: BigNumberish; b: BigNumberish; c: boolean },
overrides?: CallOverrides
): Promise<[BigNumber, BigNumber] & { d: BigNumber; e: BigNumber }>;

callStatic: {
test(
input: { a: BigNumberish; b: BigNumberish; c: boolean },
overrides?: CallOverrides
): Promise<[BigNumber, BigNumber] & { d: BigNumber; e: BigNumber }>;
};
...
```

Which isn't ideal for several reasons. One of the biggest problems that it's not possible to write function like:

```ts
let x = prepareArg();
contract.test(x);
```

Because `prepareArg` should contain `struct Input` as part of signature but it's not generated.

I propose generation changed to:

```ts
export type Input = { a: BigNumberish; b: BigNumberish; c: boolean };
export type Output = { d: BigNumber; e: BigNumber };

...
test(
input: Input,
overrides?: CallOverrides
): Promise<[BigNumber, BigNumber] & Output>;

callStatic: {
test(
input: Input,
overrides?: CallOverrides
): Promise<[BigNumber, BigNumber] & Output>;
};
...
```

I can try implementing it if you're agreed this is the way to go.