Shall we introduce -Werror flag?

[radeusgd]

I've just found out (I think not for the first time) that we have code in Enso Standard Library that yields compiler warnings.

I think it would be useful if we could make our CI fail if there are any warnings in one of our Standard Libraries (or tests as well). Of course by default warnings should not be fatal, but if we can make them fatal upon request, we could ensure that we will not accidentally merge PRs with warnings in the code.

Of course the name of the CLI argument may be something else, I just used -Werror as a well-known example.