Clean up ready for null safety

[timsneath]

This PR cleans up the codebase a little:

• Upgrades to a newer version of pedantic, removes redundant rules, and fixes newly-added lints
• Uses final where possible for performance
• Fixes filename spelling nit (granulartity -> granularity)
• Uncomments out test now that bug has been fixed, and remove unnecessary test pump
• Takes advantage of Dart int-to-double inference for fonts and pixel padding

Assuming you're happy to merge this pull request, I have a second one that builds on it and adds support for sound null safety:
https://github.com/timsneath/auto_size_text/pull/1

[simc]

Great, thanks for your work!