Add Bitraverse, fixes #800

[adelbertc]

Review by @travisbrown

[codecov-io]

Current coverage is 89.39%

Merging #903 into master will decrease coverage by -0.13% as of d9c49ac

@@            master    #903   diff @@
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  Files          177     181     +4
  Stmts         2397    2423    +26
  Branches        76      76       
  Methods          0       0       
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+ Hit           2146    2166    +20
  Partial          0       0       
- Missed         251     257     +6

Review entire Coverage Diff as of d9c49ac

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[adelbertc]

Realized last night I sorta need this for my talk next week, please review soon :D

[travisbrown]

👍, looks great to me, but where's my bitraverseU? 😄

[adelbertc]

@travisbrown I tried! I ran into a problem when I tried something like:

def bitraverseU[A, B, GC, GD](fab: F[A, B])(f: A => GC, g: B => GD)(implicit ULeft: Unapply[Applicative, GC], URight: Unapply[Applicative, GD]) = ???

and I needed to make sure ULeft.M was the same as URight.M. Maybe I'm encoding it wrong.. halp :-)

[travisbrown]

@adelbertc That was kind of a joke—I've got a huge blog post on exactly that problem. My fix is in Scalaz, but I'm not sure it's worth it.

[adelbertc]

@travisbrown ah :-) on one hand I'm glad to see I wasn't crazy, on the other hand I'm also kind of sad

[non]

Looks good to me. 👍