Switch vector to use size_t for its array sizes and indices.

include/vec.h

@@ -18,11 +18,11 @@ struct vec_s {
   /**
    * The maximum size of the vector.
    */
-  int capacity;
+  size_t capacity;
   /**
    * The current number of items in the vector.
    */
-  int size;
+  size_t size;
 };
 
 typedef struct vec_s vec;
@@ -87,7 +87,7 @@ int vec_foreach(vec* vec, void* data, compare_func compare, foreach_func foreach
  * @param index the index to retreive a value from
  * @return pointer to the value, or, if the index is out of bounds (or there is nothing there) NULL.
  */
-void* vec_index(vec* vec, int index);
+void* vec_index(vec* vec, size_t index);
 /**
  * Clears the vector, removing all elements from it
  * but not resizing it down.