Warn if a variable is never changed

[atomicbitty]

It would be nice if the compiler warned us about vars that are never modified so that we can make them consts.

[tauoverpi]

Already covered by #224 though it would be slightly annoying when playing around with code samples if #3320 isn't implemented.

[atomicbitty]

In #224 the compiler would throw an error for that case, but I think an error is unnecessary and very annoying for this situation, that's why I think a warning would suffice.

[atomicbitty]

Apparenty zig doesn't have warnings, even though I don't think this is something good...