feat(darr): add darr_clear

include/c-libs/dyn-arr.h

@@ -76,6 +76,10 @@ size_t darr_cap(void *arr);
 /// Free a dynamic array
 /// @param arr a pointer initialized with darr_init or returned from darr_clone
 void darr_free(void *arr);
+
+/// Empty the dynamic array
+void darr_clear(void *arr);
+
 #endif
 
 #ifdef CLIBS_DYN_ARR_IMPL
@@ -103,4 +107,10 @@ void darr_free(void *arr) {
   DArrHeader *header = ((DArrHeader *)arr) - 1;
   free(header);
 }
+
+void darr_clear(void *arr) {
+  DArrHeader *header = ((DArrHeader *)arr) - 1;
+  header->size = 0;
+}
+
 #endif