Dune environment variables suppressing compiler warnings

[MartyO256]

The following dune environment variables are set in the core, harpoon and optparser packages. These globally disable compilation warnings such as unused variables. However, what I find alarming is that warnings about not exhaustive pattern-matching are also suppressed. This explains why there are not easily reproducible pattern-matching errors thrown in Harpoon sessions. https://github.com/Beluga-lang/Beluga/blob/3db1ffd08d4c3bde7ad2ceb924bfb95488eef2b2/src/core/dune#L7-L9 https://github.com/Beluga-lang/Beluga/blob/3db1ffd08d4c3bde7ad2ceb924bfb95488eef2b2/src/harpoon/dune#L6-L8 https://github.com/Beluga-lang/Beluga/blob/3db1ffd08d4c3bde7ad2ceb924bfb95488eef2b2/src/optparser/dune#L3-L5

Where not exhaustive pattern-matching is intended as in the following, then warnings should be explicitly suppressed with macros like [@warning "-8"] instead of being toggled off globally. https://github.com/Beluga-lang/Beluga/blob/3db1ffd08d4c3bde7ad2ceb924bfb95488eef2b2/src/core/error.ml#L116-L122

Fixing this issue won't be easy as it affects most files.

[Ailrun]

There's really a lot of non-exhaustive pattern matches, so it's hard to enable the warning. An affordable solution would be handling them one-by-one (though it will still take long time)