Linters needed

[fare]

• [ ] Linter for imports: any import that isn't somehow referenced or exported should be flagged. Minimal (import (only-in ...) ...) forms should be offered as replacement for those currently used.
• [ ] Linter for Gambit symbols neither exported nor flagged as not to export.
• [ ] Linter for documentation and tests. Every exported symbol should be mentioned in documentation and in tests. For documentation, it could be that of a reexport. For test, it should be in a test for the defining module.