Generate interfaces for structs alongside existing struct types

PR https://github.com/dethcrypto/TypeChain/pull/700 changed how struct types are generated. I've been importing these structs (before the properties were wrapped by `PromiseOrValue`) to help me type check inputs in hardhat tests.

Perhaps alongside the existing struct types you could provide an `interface` type also:

```
export interface Foo {
a: BigNumberish;
b: BigNumberish;
};

export type FooStruct = {
a: PromiseOrValue;
b: PromiseOrValue;
};

export type FooStructOutput = [BigNumber, BigNumber] & {
a: BigNumber;
b: BigNumber;
};
```