upgrade to TZ DB version 2020a

• 1.1 (2020-04-25, TZ DB version 2020a)
  • Fix broken links in README.md.
  • Fix typos in USER_GUIDE.md and update its version number to 1.0 as it
    should have been back in October.
  • Massive refactor of ./tools processing pipeline and update
    validation/tests.
    • Add mypy strict type checking for Python scripts under tools.
    • Funnel validation*.{h,cpp} code generation through a single program
      using a validation_data.json intermediate file.
    • Funnel processing of TZDB output from transformer.py into a single
      tzdbcollector.py which can produce a tzdb.json output.
    • Separate validator.py processing into a distinct section.
  • Add validation tests against Python dateutil library (similar to
    pytz).
  • Update TZ Database version to 2020a that was released on 2020-04-23.