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/array_string.rs

@@ -9,7 +9,7 @@ use std::str::FromStr;
 use std::str::Utf8Error;
 use std::slice;
 
-use array::{Array, ArrayExt};
+use array::Array;
 use array::Index;
 use CapacityError;
 use char::encode_utf8;