HashMap trees generated in property tests are too flat and too small

[sjakobi]

unordered-containers/tests/Strictness.hs

Lines 24 to 29 in 4da2c20

	-- Key type that generates more hash collisions.
	newtype Key = K { unK :: Int }
	deriving (Arbitrary, Eq, Ord, Show)
	instance Hashable Key where
	hashWithSalt salt k = hashWithSalt salt (unK k) `mod` 20

This hash function produces only values in the range [0..19]. As a result, a typical generated tree will have a BitmapIndexed root node that contains some Leafs and some Collisions. No Full nodes at all, and no internal/non-root BitmapIndexed nodes!

It seems very likely that some code added since #317 hasn't been tested at all. :/

[sjakobi]

Maybe it would also be good to deduplicate the Key type declarations. Currently, tests/Properties/HashMapLazy.hs, tests/Properties/HashSet.hs and tests/Strictness.hs all contain the same newtype Key declaration.