Get doctests into testsuite

It seems like a bit of a shame not to have Mr.Orlitzky's lovely doctests not running with the testsuite. Or perhaps to at least have some sort of make doctest. If there is some interest to get this working, I'd like to take a look at submitting a patch.

Here are the previous documentation patches I can find: https://phabricator.haskell.org/p/mjo/

I can see the following files that have doctests:

ruby-2.2.1 ~/work/contributing/ghc (master) [12:35:44]
lwm$ grep -R "=== __Examples__" libraries/base/ | uniq
grep: libraries/base/Data/Maybe.hs:-- ==== __Examples__
libraries/base/Data/Either.hs:-- ==== __Examples__
libraries/base/Data/List.hs:-- ==== __Examples__
libraries/base/Data/Functor.hs:-- ==== __Examples__
libraries/base/Data/Bool.hs:-- ==== __Examples__
libraries/base/Data/Char.hs:-- ==== __Examples__
libraries/base/GHC/Unicode.hs:-- ==== __Examples__
libraries/base/Text/Printf.hs:-- ==== __Examples__

Trac metadata

Trac field	Value
Version	7.10.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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CC	ekmett
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