Switch to using calloc instead of malloc

dstr/src/dstr.c

@@ -15,15 +15,13 @@ struct dstr {
 };
 
 String *wapp_dstr_with_capacity(u64 capacity) {
-  String *out = (String *)malloc(sizeof(String) + capacity + 1);
-
+  String *out = (String *)calloc(1, sizeof(String) + capacity + 1);
   if (!out) {
     return NULL;
   }
 
   out->capacity = capacity;
   out->size = 0;
-  memset(out->buf, 0, capacity + 1);
 
   return out;
 }