Define laws for Traversable #373

[Badmoonz]

closes #373

[sideeffffect]

Absolutely awesome job!
I've just one small nit-pick, and if @adamgfraser doesn't have anything to add, I think we can merge 🚀 (but before doing that, we should coordinate somehow with the twin PR #449 )
The jobs are restarted, so 🤞 it will be all green now 🍀

[sideeffffect]

I would be worried that the Haskell names could confuse some users/contributors. I think we should use the ZIO Prelude terminology here, so foreach (instead of traverse) and map (instead of fmap).

[sideeffffect]

Looking at the builds again, some were not spurious failures. 2.11 may need more explicit type arguments and dotty seems to need more parentheses.
Do you think you could have a look at it @Badmoonz 🙏

[zalbia]

@Badmoonz I tried adding tests for nested (and product of) Covariant & IdentityBoth` like you said, and ended up with the same 2.11 errors. I don't know how to fix it myself.