ocaml-pds: bind put-and-get example as a function
The block referenced [record_bytes] as a free variable; wrap as
[put_and_get ~record_bytes = ...] so the snippet is self-contained.
Also add [eio.core] / [eio_main] / [fmt] / [atp] to the mdx libraries
so [Atp.Did] / [Eio.Switch] / [Fmt.pr] resolve.