make Unapply serializable

[kailuowang]

This will enable typeclasses that uses Unapply to be serializable themselves.

[ceedubs]

Thanks, @kailuowang. Could you please add a test (probably something like

cats/tests/src/test/scala/cats/tests/FunctionTests.scala

Line 19 in 9780e8f

checkAll("Bimonad[Function0]", SerializableTests.serializable(Bimonad[Function0]))

)?

[kailuowang]

@ceedubs test added.

[adelbertc]

👍 on Travis green

[codecov-io]

Current coverage is 89.59%

Merging #905 into master will increase coverage by +0.04% as of 9f5602a

@@            master    #905   diff @@
======================================
  Files          181     181       
  Stmts         2423    2423       
  Branches        76      76       
  Methods          0       0       
======================================
+ Hit           2170    2171     +1
  Partial          0       0       
+ Missed         253     252     -1

Review entire Coverage Diff as of 9f5602a

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[ceedubs]

👍 thanks!