Improve performance of computation of `sizeTxF` for `ShelleyTx` and `AlonzoTx`.

There should be no reason why `ShelleyTx` retains the whole of original bytes, because it can't do so in a block anyways. The only reason why it uses `MemoBytes` is for size calculation needed for `sizedTxF`. Remove usage of `MemoBytes` for `ShelleyTx`, thus avoiding keeping around bytes for . However in order for us to memoize the size needed for computing the fee we should add a field to `AlonzoTx` and `ShelleyTx`. This will require creation of `AlonzoTxRaw` and removal of `MemoBytes` for `ShelleyTx`