API: Update ArrayExt's methods to expose slices instead of pointers

This is a nicer and simpler interface to expose, since the ref to uninit
array to pointer cast is not usable anyway (it's historic now, arrayvec
itself does not use it anymore).

src/lib.rs

@@ -155,7 +155,7 @@ impl<A: Array> ArrayVec<A> {
     /// assert_eq!(array.capacity(), 3);
     /// ```
     #[inline]
-    pub fn capacity(&self) -> usize { A::capacity() }
+    pub fn capacity(&self) -> usize { A::CAPACITY }
 
     /// Return if the `ArrayVec` is completely filled.
     ///