Add send_describe_prepared and send_describe_portal and test.

I'm adding two more asynchronous functions: send_describe_prepared and send_describe_portal.  Hopefully I got the stub code right this time; at least no allocations to worry about.  I also added a program to the examples directory to test various async functions.  I realized that directory may be intended for human readable example code rather than test cases, so let me know if it's a problem.