Move NaturalTransformation.or from companion to trait

[ceedubs]

I think this provides slightly nicer syntax for combining natural
transformations, and it makes it more consistent with andThen and
compose. If people like it, they can merge, but if not it's no
problem.

[ceedubs]

@raulraja as the original contributor of or, what do you think?

[raulraja]

LGTM! Also or as infix in the examples looks so much cleaner

[codecov-io]

Current coverage is 83.89%

Merging #697 into master will not affect coverage as of 9ceaef6

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Review entire Coverage Diff as of 9ceaef6

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[lvicentesanchez]

I like it. That would help me to get rid of my own implicit/explicit or that I use to combine more than 2 NT.

[non]

👍

[adelbertc]

👍