Implement Exec::wait_any on top of ActivitySet and deprecate one method

Implement Exec::wait_any on top of ActivitySet and deprecate one method

- Also fix a bit the behavior when ActivitySet::wait_any_for() results
  in a timeout. Believe it or not, this was not tested and plainly wrong
  until now.
  Now, it's a bit too complex, as the simcall returns -1 and forces
  the caller to detect it and throw TimeoutException manually. It
  should be cleaned as in wait_for, somehow.
- The C API is not deprecated yet, because ActivitySet has no C API yet
- The C++ API is not fully deprecated yet, because the unit test is
  not converted to ActivitySet yet.

Still a long way to go to clean it up :(