Add ability to asynchronously wait for a duration of time

[lewissbaker]

Add the ability to do something like:

cppcoro::task<> do_something(cppcoro::io_context ioCtx)
{
  using namespace std::chrono_literals;
  co_await ioCtx.schedule_after(30s);
}

[lewissbaker]

I'm working on support for this at the moment for the io_service class.

I'm also adding the ability to cancel a timer using a cancellation_token.

using namespace std::chrono_literals;
cppcoro::task<> do_something(cppcoro::io_service& io, cancellation_token ct)
{
  // throws cppcoro::operation_cancelled if cancellation is requested before timer elapses.
  co_await io.schedule_after(30s, ct); 
}

[lewissbaker]

830c4d1 has implemented this for io_service on the Windows platform.

I've also added support for cancellation of timers by passing an optional cancellation_token into the schedule_after method.

I'm still working on adding some documentation for io_service and schedule_after() to the README.

[lewissbaker]

I've now added some basic docs to the README