Implement KeyMap.!? operator.

This is to match the counterpart in HashMap, making it smoother
migration to aeson-2+

Author: Javran

src/Data/Aeson/KeyMap.hs

@@ -16,6 +16,7 @@ module Data.Aeson.KeyMap (
     -- * Query
     null,
     lookup,
+    (!?),
     size,
     member,
 
@@ -184,6 +185,14 @@ alterF f k m = fmap g (f mv) where
 lookup :: Key -> KeyMap v -> Maybe v
 lookup t tm = M.lookup t (unKeyMap tm)
 
+-- | Return the value to which the specified key is mapped,
+-- or Nothing if this map contains no mapping for the key.
+--
+-- This is a flipped version of 'lookup'.
+--
+(!?) :: KeyMap v -> Key -> Maybe v
+(!?) m k = lookup k m
+
 -- | Associate the specified value with the specified key
 -- in this map. If this map previously contained a mapping
 -- for the key, the old value is replaced.
@@ -387,6 +396,14 @@ alterF f k = fmap KeyMap . H.alterF f k . unKeyMap
 lookup :: Key -> KeyMap v -> Maybe v
 lookup t tm = H.lookup t (unKeyMap tm)
 
+-- | Return the value to which the specified key is mapped,
+-- or Nothing if this map contains no mapping for the key.
+--
+-- This is a flipped version of 'lookup'.
+--
+(!?) :: KeyMap v -> Key -> Maybe v
+(!?) m k = lookup k m
+
 -- | Associate the specified value with the specified key
 -- in this map. If this map previously contained a mapping
 -- for the key, the old value is replaced.