Str\join takes readonly

Summary: Technically, traversal over readonly non-Containers isn't correct, but we're ignoring that for now -- analogous to how we're pretending traversal is always pure.

Reviewed By: jamesjwu

Differential Revision: D32027704

fbshipit-source-id: 220e9c7bb46e08fab7111d2b1d4e40d76aa8b90a

Author: viratyosin

src/str/combine.php

@@ -19,11 +19,11 @@
  * Previously known as `implode` in PHP.
  */
 function join(
-  Traversable<arraykey> $pieces,
+  readonly Traversable<arraykey> $pieces,
   string $glue,
 )[]: string {
   if ($pieces is Container<_>) {
     return \implode($glue, $pieces);
   }
-  return \implode($glue, Vec\cast_clear_legacy_array_mark($pieces));
+  return \implode($glue, vec($pieces));
 }