Add Validated.andThen

[ceedubs]

Fixes #604.

This is essentially flatMap for Validated, but since flatMap
suggests monadic bind in Cats and this is not consistent with ap,
another name is being used.

This PR is not complete yet. I need to add a Scaladoc comment and tut documentation about this change. However, I figured there might be some discussion about the name of the method, so I want to wait until a name is decided to start writing docs :). Is andThen too overloaded?

[codecov-io]

Current coverage is 76.04%

Merging #611 into master will decrease coverage by -0.25% as of 249d37c

@@            master    #611   diff @@
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  Files          160     159     -1
  Stmts         2194    2066   -128
  Branches        68      69     +1
  Methods          0       0       
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- Hit           1674    1571   -103
  Partial          0       0       
+ Missed         520     495    -25

Review entire Coverage Diff as of 249d37c

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[adelbertc]

andThen is fine by me, but it might confuse people seeing andThen from FunctionN. I know Scoobi used mapFlatten, or at least did at some point..

[mpilquist]

👍 I'm okay with andThen.

[non]

👍 I would be fine with andThen.

In the context of Validated I could also imagine calling it something about valid e.g. whenValid, mapValid, etc.

[ceedubs]

It sounds like people are generally okay with andThen, so I'll go with that. We can reconsider in the future if it's tripping people up.

I'll move forward with documentation.

[ceedubs]

I added some documentation.

@adelbertc I deleted part of your Validated docs about flatMap, because it didn't seem to belong after the andThen docs, but I'd like your input on this change.

I think there's still room for improvement, so please feel free to chime in.

[adelbertc]

LGTM 👍