add cvector_init (#51)

* add cvector_init

* formatting

cvector.h

@@ -108,6 +108,21 @@ typedef struct cvector_metadata_t {
         }                                        \
     } while (0)
 
+/*
+ * @brief cvector_init - Initialize a vector.  The vector must be NULL for this to do anything.
+ * @param vec - the vector
+ * @param capacity - vector capacity to reserve
+ * @param elem_destructor_fn - element destructor function
+ * @return void
+ */
+#define cvector_init(vec, capacity, elem_destructor_fn)               \
+    do {                                                              \
+        if (!(vec)) {                                                 \
+            cvector_reserve((vec), capacity);                         \
+            cvector_set_elem_destructor((vec), (elem_destructor_fn)); \
+        }                                                             \
+    } while (0)
+
 /**
  * @brief cvector_erase - removes the element at index i from the vector
  * @param vec - the vector