`setTransactionMessageFeePayer()` does not delete signer if one is present.

[steveluscher]

Overview

Presume that you have a TransactionMessage with a feePayerSigner. If the fee payer address changes, setTransactionMessageFeePayer() won't blank out the pre-existing feePayerSigner.

Description of bug

This may cause problems if I go from:

• Loris(A)/Phantom, to
• Loris(B)/NO_SIGNER

The Phantom feePayerSigner will still be on the message.

[steveluscher]

We could fix this once and for all by making feePayer a discriminated union.

export interface ITransactionMessageWithFeePayer<TAddress extends string = string> {
    readonly feePayer: Readonly<{__type: 'address', address: Address<TAddress>}>;
}
export interface ITransactionMessageWithFeePayerSigner<
    TAddress extends string = string,
    TSigner extends TransactionSigner<TAddress> = TransactionSigner<TAddress>,
> {
    readonly feePayer: Readonly<{__type: 'signer'}> & TSigner;
}

That would be a whole bunch of surgery, but we'd end up with something that always had address on it as we have now, no matter if it was a signer or not, and the decision of whether to overwrite it could involve checking that the two __type properties match.