The Account class has a function, createSubaccount.

I propose we rename it, safely with deprecation, etc, to createMuxedAccount.

The function name createSubaccount makes assumptions about how muxed accounts will be used. Muxed accounts are intended to solve a variety of problems, and only one of them is the ability to represents virtual accounts. They're also intended to support invoicing and really any memo transaction case where the receiver or sender wish to track a specific transaction.