Doctest examples should actually be tested

[sjakobi]

It would be nice if our doctests such as these for compose were automatically checked.

unordered-containers/Data/HashMap/Internal.hs

Lines 1695 to 1696 in 10328f2

	-- >>> compose (fromList [('a', "A"), ('b', "B")]) (fromList [(1,'a'),(2,'b'),(3,'z')])
	-- fromList [(1,"A"),(2,"B")]

We'd probably have to pin hashable for this test-suite, since the example output depends on the exact hash implementation.