This is a simple example of reading/writing mojo messages adhering to the Echo
protocol: https://github.com/domokit/mojo/blob/master/examples/echo/echo.mojom.
The program assumes that it starts up with a handle value to a message pipe and
that something on the other end of that message pipe is sending some number of
Echo calls along the pipe. The program:
*) waits for the message pipe to be readable
*) reads an incoming message
*) validates that it seems sane
*) formats a reply message
*) sends the reply
*) goes back to waiting

until the message pipe peer is closed or any other error is encountered. The
serialization/deserialization is hand written as we don't have code generation
for mojom set up for C yet.
