[playground] Error: Reference to undefined global

[hyphenrf]

It looks like the Bool module is not currently loaded in https://ocaml.org/play

Repro:

• open the playground
• type in an expression that uses a Stdlib.Bool function, for example Bool.not true
• run
• error message is Error: Reference to undefined global `Stdlib__Bool'

Investigation:

• tested and failed on ocaml.org under both Firefox and Chromium
• tested and worked on oscigen's toplevel
• tested other basic types. found the following also missing: Unit, Either, Weak, ... there may be others
• could be related: #908