add an initialize method to Heap

[japaric]

this lets you create an allocator that doesn't depend on lazy_init. Example below:

static HEAP: Mutex<Heap> = Mutex::new(Heap::new());

unsafe fn before_main() {
    HEAP.lock().init(BOTTOM, SIZE);
}

fn main() {
    // Use the allocator
}

[phil-opp]

Thanks!