More transaction related helper functions

[KristianBalaj]

Currently there are just 3 helper functions in the aiken/transaction module (i.e. find_script_outputs, find_datum, find_input).

I'm proposing to add further helper function inspired by PlutusTx:

• findOwnInput - https://input-output-hk.github.io/plutus/master/plutus-ledger-api/html/PlutusLedgerApi-V2-Contexts.html#v:findOwnInput - Resolves #44
• getContinuingOutputs - https://input-output-hk.github.io/plutus/master/plutus-ledger-api/html/PlutusLedgerApi-V2-Contexts.html#v:getContinuingOutputs
• txSignedBy - https://input-output-hk.github.io/plutus/master/plutus-ledger-api/html/PlutusLedgerApi-V2-Contexts.html#v:txSignedBy
• checkOwnOutput - check whether there's an output with expected datum and value going to the same script as currently being validated

I will be adding further items to this list but at least these for now. I will be also adding PRs for those functions over time.

[rvcas]

Something to consider perhaps

#45

[micahkendall]

I suggest that

txSignedBy :: [TxInfo] -> [PubKeyHash] -> [Bool]

is instead

signedByAll :: [PubKeyHash] -> [PubKeyHash] -> [Bool]

or some other variant. Once there are type classes etc. it can be an alias for list subset.

[micahkendall]

I don't like getContinuingOutputs/checkOwnOutput as they seem to be unnecessarily limiting in many cases, or encourage bad habits. Maybe we should have a compatibility library for all these ideas, and encourage datum/nft tagging etc. as alternatives.