feat: input indexers

[mpetruska]

Input indexers are highly used optimization strategies in most of Cardano's protocols. See more details at https://developers.cardano.org/docs/build/smart-contracts/advanced/design-patterns/utxo-indexers .
Example:

...
  input input1 { ... }
  input input2 { ... }
  input validator_input {
    from: MyValidator,
    redeemer: MyValidator::MyRedeemer {
      ix: index_of(input1)
    }
  }
...