`locals()` and `globals()` messes with scope checking

There are three mutable, globally accessible dicts called `locals()` and `globals()`, and `vars()`. When these dicts are modified, new variables are brought into scope. Warn about these usages. Warnings should only occur when it's the built-in definitions that are accessed. If a variable called `globals` is introduced, shadowing the original, then there's no issue.