async output

This release:

• adds OutputInterface::writeAsync(string): Awaitable<void> that will write the complete provided buffer
• adds OutputInterface::rawWrite(string): int, which returns the number of bytes written
• adds CLIBase::displayHelpAsync(OutputInterface): Awaitable<void> to replace CLIBase::displayHelp

OutputInterface::write() and CLIBase::displayHelp() are still present, but will be removed in a future release.