Fix periodic timers firing only once

[michael-p]

Creating a Timer with auto_reload set to true (the default) results in the timer callback being invoked just once, not periodic.

The problem is that the Drop implementation of Timer stops the timer, and since the timer_callback function constructs a Timer out of thin air and passes that by value to the timer callback Box<dyn Fn(Timer)> the timer gets dropped when the callback ends and stops. Relevant code: https://github.com/lobaro/FreeRTOS-rust/blob/master/freertos-rust/src/timers.rs#L120

This was introduced in #57. Previously, Timer had a detached flag that prevented it from being stopped in its destructor in this situation. Instead of trying to revert this this PR changes the API so that timer callbacks get passed in the Timer by reference. I think this makes more sense than giving ownership of the timer to its own callback.