avoid tuple allocations in cmd_args.add()

Summary: We call `cmd_args.add()` a LOT. Using an `UnpackTuple` forces `starlark` to allocate a tuple on the heap every time it's called. By processing the arguments ourselves we avoid the `heap.alloc_tuple`, and reduce the overall allocations occuring on starlark `Heap`s

Reviewed By: JakobDegen

Differential Revision: D69211163

fbshipit-source-id: 3a10ec66e1ed8ef3fbed69628a986fb0f4be59ba

Author: Jeremy Braun

starlark/src/eval/runtime/arguments.rs

@@ -346,7 +346,7 @@ impl<'v, 'a> Arguments<'v, 'a> {
     }
 
     /// Unpack all positional parameters into an iterator.
-    pub(crate) fn positions<'b>(
+    pub fn positions<'b>(
         &'b self,
         heap: &'v Heap,
     ) -> crate::Result<impl Iterator<Item = Value<'v>> + 'b> {