Support remaining Stdlib modules

[anmonteiro]

From #710, these modules don't currently work in Melange. It'd be nice to add support for (at least some of) them eventually:

• Bigarray
• Callback
• Condition
• Effect
• Semaphore
• Ephemeron
• Gc
• In_channel
• Marshal
• Nativeint
• Out_channel
• Weak

[ELLIOTTCABLE]

Mentioned in Slack, but WeakRef :: Weak.t, and WeakMap :: Ephemeron, ish.

Presumably lots of subtle specification differences, though … unsure how realistic a mapping for those two is.