Move Transaction decoding out of Client

[epost]

This stuff, currently in stbx-client-http:

newtype DecodingError = DecodingError String

instance showDecodingError :: Show DecodingError where
  show = case _ of
    DecodingError e -> "(DecodingError " <> show e <> ")"

decodeTxSum :: HashStr -> Json -> DecodingError \/ TxSum
decodeTxSum hash json =
  lmap DecodingError (decodeWiring json <|> decodeFiring json)
  where
    decodeWiring :: Json -> String \/ TxSum
    decodeWiring json = WiringTxInj <<< _.decoded <$> decodeJson json :: String \/ Tx WiringTx

    decodeFiring :: Json -> String \/ TxSum
    decodeFiring json = FiringTxInj <<< _.decoded <$> decodeJson json :: String \/ Tx FiringTx

should probably be somewhere under theCore module. I wanted to try and keep it separate initially (one reason being the current argonaut dependency) but the HTTP API module is not the right place for this.