Increase overall strictness of compiler #685

New guidelines:

• If it's purely a subjective style issue, then it should only ever show up on --strict.

• If it's a general code cleanliness issue with no syntax for dismissing the issue, then it should only ever show up as a warning.

• If it's a general code cleanliness issue that has a syntax for dismissing the issue, then it should be a warning regardless and an error on --strict.

• If it's a deprecated feature, it should be a warning regardless and an error on --strict.