Allow instance Typeable in withStatic blocks

`Typeable` instances are not user defined. But the user nevertheless
sometimes needs static evidence of typeability. The solution is to
allow users to write

```haskell
withStatic [d| instance Typeable T |]
```

for any datatype `T`. `withStatic` will generate static evidence, but
won't pass through the instance declaration itself, since it is
illegal.

examples/ClientServer.hs

@@ -82,7 +82,7 @@ double = (*2)
 newtype SerializableInt = SI Int deriving (Generic, Typeable)
 withStatic [d|
   instance Binary SerializableInt
-  instance Serializable SerializableInt
+  instance Typeable SerializableInt
   |]
 
 -- | Demonstration of client server interactions.

src/Control/Distributed/Closure/TH.hs

@@ -92,7 +92,9 @@ mangleName name@(TH.Name occ fl) = case fl of
 -- @
 --
 -- You will probably want to enable @FlexibleContexts@ and @ScopedTypeVariables@
--- in modules that use 'withStatic'.
+-- in modules that use 'withStatic'. 'withStatic' can also handle non-user
+-- generated instances like 'Typeable' instances: just write @instance Typeable
+-- T@.
 withStatic :: TH.DecsQ -> TH.DecsQ
 withStatic = (>>= go)
   where
@@ -140,5 +142,9 @@ withStatic = (>>= go)
         let staticins = TH.InstanceD staticcxt statichd methods
 #endif
         decls' <- go decls
-        return (ins : sigf : declf : staticins : decls')
+        case hd of
+          TH.AppT (TH.ConT nm) _ | nm == ''Typeable ->
+            return (sigf : declf : staticins : decls')
+          _ ->
+            return (ins : sigf : declf : staticins : decls')
     go (decl:decls) = (decl:) <$> go decls