Refactor Gavel validators (non-breaking)

[artem-zakharchenko]

Gavel internals can be refactored, preserving the same public API.

Why

• To have a more readable code
• To have less code
• To remove classes and mutations

Roadmap

• Modularize validation (Modularize validation #155)
• Rewrite validators
  • Use weak comparison for statusCode validation #159
  • Gavel.js should print an invalid media type #88

[artem-zakharchenko]

Removing the milestone because the remaining changes are not breaking, and are entirely internal.

[artem-zakharchenko]

Gavel validators partially refactored during #90. Now all validators are constructed with only expected data, and any actual data is accepted as an argument during the validator.validate() call.

There is still a room for improvement (i.e. obscure class extensions for HeadersExample), yet none of them are blocking.