Add support for tracking unknown fields. (google#129)

- Every message has a new field containing a list of unknown fields:
  ```unknownFields :: Message msg => Lens' msg [TaggedValue]```
- Unknown fields are preserved by `decodeMessage`, `encodeMessage`,
  and `showMessage`
- Unknown fields still cause an error for `readMessage`.

A few TODOs:
- For now, unknown groups are printed sub-optimally by `showMessage`: the
  start/end group tags (and everything in between) all get displayed as
  individual fields, rather than being organized into a sub-struct.
- The `discardUnknownFields` function isn't recursive, unlike in other
  languages.
- The Ord instance doesn't try to do anything special, just treating
  the unknown fields as a list of values.  If it really matters then
  `discardUnknownFields` can help resolve the ambiguity.

src/Data/ProtoLens/Compiler/Definitions.hs

@@ -92,6 +92,9 @@ data MessageInfo n = MessageInfo
     , messageOneofFields :: [OneofInfo]
       -- ^ The oneofs in this message, associated with the fields that
       --   belong to them.
+    , messageUnknownFields :: Name
+      -- ^ The name of the Haskell field in this message that holds the
+      -- unknown fields.
     } deriving Functor
 
 -- | Information about a single field of a proto message.
@@ -225,6 +228,8 @@ messageDefs protoPrefix hsPrefix d
                   map (fieldInfo hsPrefix')
                       $ Map.findWithDefault [] Nothing allFields
             , messageOneofFields = collectOneofFields hsPrefix' d allFields
+            , messageUnknownFields =
+                  fromString $ "_" ++ hsPrefix' ++ "_unknownFields"
             }
 
 fieldInfo :: String -> FieldDescriptorProto -> FieldInfo

src/Data/ProtoLens/Compiler/Generate.hs

@@ -159,6 +159,7 @@ generateMessageDecls syntaxType env protoName info =
                   [ (recordFieldName f, recordFieldType f)
                   | f <- allFields
                   ]
+                  ++ [(messageUnknownFields info, "Data.ProtoLens.FieldSet")]
         ]
         ["Prelude.Show", "Prelude.Eq", "Prelude.Ord"]
     ] ++
@@ -221,7 +222,9 @@ generateMessageDecls syntaxType env protoName info =
                       [ fieldUpdate (unQual $ haskellRecordFieldName $ oneofFieldName o)
                             "Prelude.Nothing"
                       | o <- messageOneofFields info
-                      ]
+                      ] ++
+                      [ fieldUpdate (unQual $ messageUnknownFields info)
+                            "[]"]
             ]
         ]
     -- instance Message.Message Bar where
@@ -691,6 +694,7 @@ descriptorExpr syntaxType env protoName m
           @@ ("Data.Text.pack" @@ stringExp (T.unpack protoName))
           @@ ("Data.Map.fromList" @@ list fieldsByTag)
           @@ ("Data.Map.fromList" @@ list fieldsByTextFormatName)
+          @@ rawFieldAccessor (unQual $ messageUnknownFields m)
   where
     fieldsByTag =
         [tuple