addition/refactor: Made ADT design and added grow/shrink

So, added private functions made to grow and shrink the array in a
multiple of 2, 2 as a factro is fine, at least that is Rust does it that
way. Also changed the design to ADT, forgot i liked that, probably will
be changing that to apply also to my Arena implementation, i will see if
fits --probably--.

include/arraylist.h

@@ -5,15 +5,10 @@
 #include <stddef.h>
 #include <stdbool.h>
 
-typedef struct {
-    void *data;
-    size_t capacity;
-    size_t len;
-    size_t elem_size;
-} ArrayList;
+typedef struct ArrayList ArrayList;
 
 typedef struct {
-    ArrayList array;
+    ArrayList *array;
     size_t offset;
 } ArrayListSlice;
 
@@ -27,21 +22,13 @@ typedef enum {
     ARRLIST_INVALID_ELEM_SIZE,
 } ArrayListErr;
 
-typedef struct {
-    bool is_valid;
-    union {
-        ArrayListErr err;
-        ArrayList array;
-    };
-} ArrayListResult;
-
-ArrayListResult arraylist_init(size_t capacity, size_t elem_size);
+ArrayList *arraylist_init(size_t capacity, size_t elem_size);
 ArrayListErr arraylist_destroy(ArrayList *arr);
 ArrayListErr arraylist_clear(ArrayList *arr);
 
-size_t arraylist_size(ArrayList *arr);
-size_t arraylist_capacity(ArrayList *arr);
-bool arraylist_is_empty(ArrayList *arr);
+size_t arraylist_size(const ArrayList *arr);
+size_t arraylist_capacity(const ArrayList *arr);
+bool arraylist_is_empty(const ArrayList *arr);
 
 ArrayListErr arraylist_push_back(ArrayList *arr, void *data);
 ArrayListErr arraylist_insert(ArrayList*arr, size_t index, void *data);
@@ -52,7 +39,7 @@ ArrayListErr arraylist_pop_back(ArrayList *arr, void *out);
 ArrayListErr arraylist_remove_at(ArrayList *arr, size_t index, void *out);
 ArrayListErr arraylist_pop_front(ArrayList *arr, void *out);
 
-ArrayListErr arraylist_get(ArrayList *arr, size_t index, void *out);
+ArrayListErr arraylist_get(const ArrayList *arr, size_t index, void *out);
 ArrayListErr arraylist_set(ArrayList *arr, size_t index, void *data);
 
 ArrayListErr arraylist_resize(ArrayList *arr, size_t new_capacity);