[Plugin] Warn about missing builtins

[effectfully]

This makes it so that an unhandled built-in function in the plugin causes a compile-time warning.

In particular, it gives me

warning: [GHC-62161] [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In a case alternative:
        Patterns of type ‘PLC.DefaultFun’ not matched:
            PLC.IntegerToByteString
            PLC.ByteStringToInteger

[michaelpj]

Are you also going to fix the missing builtins?

[michaelpj]

I pushed the fixes

[effectfully]

Eek, I told @kwxm I'd just wait for him to complete his work and only merge my PR afterwards as it's not important and this seemed to be the simplest strategy. But feel free to merge, I guess.

There are build errors BTW.

[michaelpj]

IDK, seems well worth it and it works now 🤷