Add a microbenchmark for coroutine create, enter, and return (aka
lifecycle).  This is a useful benchmark because users are expected to
create many coroutines, one per I/O request for example, and we
therefore need to provide good performance in that scenario.

To run:

  make test-coroutine
  ./test-coroutine --benchmark-lifecycle 20000000

This will do 20,000,000 coroutine create, enter, return iterations and
print the resulting time.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>