+S4U new features:
+ - s4u::Io: IOs go asynchronous as activities. This comes with new methods in the
+ s4u::Storage class:
+ - io_init(sg_size_t, s4u::Io::OpType) to create a READ or WRITE asynchronous
+ IO operations that can be started, waited for, or canceled as a regular
+ activity.
+ - read_async(sg_size_t) and write_async(sg_size_t) which are wrappers on
+ io_init() + start()
+