Ensure static executor and timer

[eivindbergem]

The priority queue used by the executor and timer is only safe if it is static. Because of this, functions such as polling tasks are now unsafe. By wrapping the executor in a struct that holds a static reference, we can safely call the unsafe functions from the wrapper:

impl ShouldBeStatic {
    unsafe fn danger(&mut self) {
        do_unsafe_stuff_that_is_fine_if_static();
    }
}

struct ShoudBeStaticRef {
    target: &'static mut ShouldBeStatic,
}

impl ShouldBeStaticRef {
    fn danger(&mut self) {
        unsafe { self.target.danger(); }
    }
}

[eivindbergem]

Continued in #18