async: add sleep_for sender

Sometimes it is useful to do the equivalent of
`std::this_thread::sleep_for` in a co-routine context.  Add a
sender-based implementation to async.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9991eb40b7a1b12e61511f1200bc99fdcdbccf0a
diff --git a/test/meson.build b/test/meson.build
index c2e937a..0181766 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -22,6 +22,7 @@
 tests = [
     'async/context',
     'async/task',
+    'async/timer',
     'bus/list_names',
     'bus/match',
     'event/event',