Determine === and =!= strategy

[denisrosset]

Let's group here a bunch of stuff:

294 Support Eq in tests in a shouldEqv matcher (let's have a look)

601 BaseSyntaxTest uses .equals

599 No hashCode for Number

619 Remove === and =!= methods (PR)

423 Add === method for all data types (!)

405 Drop cooperative eq. or test it?

444 Add cooperative equality tests

508 More principled approach to equality

611 Quaternion and Complex use cooperative equality

[denisrosset]

After discussion with @non:

• We keep cooperative equality/hashCode for compatibility with collections and the OO world in general.
• In parallel, we provide Eq and Hash instances.
• This defines two parallel "worlds" under composition. We have to test that the definitions match using laws.
• Data types should not have === and =!= methods as those are special. Adding methods for other operators is fine. For that, revisit @rklaehn work.
• When the Eq typeclass calls a method on the type, call the method internal_eq or something similar.
• These rules should go in the style guide.

[kschwarz1116]

It appears the only remaining tracked issue here is the === and =!= strategy.