Add syntax for separate from MonadCombine.

[peterneyens]

I based myself on the structure in syntax.bitraverse for the two implicit conversions (the
existing for unite and the new for separate).

MonadCombine separate was added in #864 by @adelbertc.

[codecov-io]

Current coverage is 90.88%

Merging #975 into master will increase coverage by +0.01% as of 04498e9

@@            master    #975   diff @@
======================================
  Files          181     181       
  Stmts         2160    2162     +2
  Branches        42      42       
  Methods          0       0       
======================================
+ Hit           1963    1965     +2
  Partial          0       0       
  Missed         197     197       

Review entire Coverage Diff as of 04498e9

Powered by Codecov. Updated on successful CI builds.

[ceedubs]

👍 looks good. Thanks!

[adelbertc]

Hm what is the motivation of calling it "nested" and having it in a MonadCombineSyntax1 as opposed to just the MonadCombineSyntax?

[peterneyens]

I followed the same structure as in the bitraverse syntax.

MonadCombineSyntax1 doesn't seem necessary yes (and neither does BitraverseSyntax1 ?)

Should I rename NestedMonadCombineOps to SeparateOps (like FlattenOps and IfMOps in the flatMap syntax?

[peterneyens]

I have renamed the NestedMonadCombineOps to SeparateOps and moved the implicit conversion into MonadCombineSyntax.

I have also added a test for MonadCombine in SyntaxTests. Am I right that this file just needs to compile, for the tests to be valid ?

[ceedubs]

👍 looks good to me. Thanks @peterneyens!

[non]

Looks good to me. 👍