Support containers-0.8 in combineEq.

This commit provides a workaround for a change in the behaviour of
`Set.from{Asc,Desc}List` w.r.t. duplicate elements, when upgrading
from `containers-0.7` to `containers-0.8`.

We can use the `K` type from `Tests.Util` to demonstrate this.

With `containers-0.7`, `Set.from{Asc,Desc}List` chooses the **_first_**
occurrence of any duplicated element:

```hs
$ cabal repl nonempty-containers-test --constraint=containers==0.7
> import qualified Data.Set as Set
> import Tests.Util
> Set.fromAscList [K 'a' 1, K 'a' 2]
fromList [K {getKX = 'a', getKY = 1}]
> Set.fromDescList [K 'a' 1, K 'a' 2]
fromList [K {getKX = 'a', getKY = 1}]
```

With `containers-0.8`, `Set.from{Asc,Desc}List` chooses the **_last_**
occurrence of any duplicated element:

```hs
$ cabal repl nonempty-containers-test --constraint=containers==0.8
> import qualified Data.Set as Set
> import Tests.Util
> Set.fromAscList [K 'a' 1, K 'a' 2]
fromList [K {getKX = 'a', getKY = 2}]
> Set.fromDescList [K 'a' 1, K 'a' 2]
fromList [K {getKX = 'a', getKY = 2}]
```

This commit adjusts the bias of `Data.Set.NonEmpty.combineEq` to match
the bias of `Set.from{Asc,Desc}List` in the version of `containers`
being linked against.

Author: jonathanknowles

src/Data/Set/NonEmpty.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections #-}
@@ -1054,5 +1055,9 @@ combineEq (x :| xs) = go x xs
   where
     go z [] = z :| []
     go z (y : ys)
+#if MIN_VERSION_containers(0,8,0)
+      | z == y = go y ys
+#else
       | z == y = go z ys
+#endif
       | otherwise = z NE.<| go y ys