Support muxed addresses (SEP-23) by default.

[Shaptic]

As part of the rollout of muxed accounts (addresses in the M... form which combine a G... address alongside a uint64 identifier), we need to make M-addresses supported by default. In #348 we enabled their support via explicit opt-in; now, developers should disable supporting them only via explicit opt-out.

This is (intentionally) a breaking change.

As a reminder, the following fields can contain a muxed account. Note that while the below list should be accurate, the XDR is always an authoritative source:

• the SourceAccount of any Operation
• the Destination of the three payment operations: PaymentOp, PathPaymentStrictReceive, and PathPaymentStrictSend
• the Destination of an AccountMergeOp
• the From of a ClawbackOp
• the SourceAccount of any Transaction
• the FeeSource of a FeeBumpTransaction