feat(queue): introduce a queue ds, internally this is just a stack...

the only actual difference is a oneline change which makes pushing
append instead of prepend the new data.

README.md

@@ -45,7 +45,7 @@ ds_hmp_free(&hmp, NULL);
 free(b);
 ```
 
-## Stack
+## Stack / Queue
 ```c
 char *a = "a";
 char *b = "b";
@@ -58,6 +58,9 @@ ds_stack_pop(stack); // "b"
 ds_stack_pop(stack); // "a"
 free(stack);
 ```
+If you need a queue just use the equivalent `ds_queue` functions and datatypes.
+Internally a queue is just a stack which pushes to the end instead of the
+beginning.
 
 # TODO
 - [ ] more data structures