Add toValidatedNel to Xor

[liff]

As a convenient shorthand for xor.toValidated.toValidatedNel.

I suppose .toValidatedNel should be the version more often used since the point of Validated is to accumulate errors?

[codecov-io]

Current coverage is 90.87%

Merging #971 into master will not affect coverage as of 95f759a

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  Files          181     181       
  Stmts         2158    2159     +1
  Branches        42      42       
  Methods          0       0       
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+ Hit           1961    1962     +1
  Partial          0       0       
  Missed         197     197       

Review entire Coverage Diff as of 95f759a

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[stew]

👍

[adelbertc]

Is the [AA >: A] needed here?

[liff]

It doesn't compile without:

cats/core/src/main/scala/cats/data/Xor.scala:75: covariant type A occurs in invariant position in type => cats.data.ValidatedNel[A,B] of method toValidatedNel
  def toValidatedNel: ValidatedNel[A,B] = fold(Validated.invalidNel, Validated.valid)
      ^

Is it due to OneAnd's invariance?

[adelbertc]

Ah yes 👍