Add support for OverloadedLabels

`OverloadedLabels` is a simple enough extension to parse and format. It
is becoming more common with use of `generic-lens`. Since it can be
treated as a `HsVar` its implementation only requires using `docLit`,
along with some marshalling for dealing with `FastString`.

src-literatetests/14-extensions.blt

@@ -130,3 +130,13 @@ func = do
   hello
   |]
   pure True
+
+###############################################################################
+## OverloadedLabels
+#test bare label
+{-# LANGUAGE OverloadedLabels #-}
+foo = #bar
+
+#test applied and composed label
+{-# LANGUAGE OverloadedLabels #-}
+foo = #bar . #baz $ fmap #foo xs

src/Language/Haskell/Brittany/Internal/Layouters/Expr.hs

@@ -55,9 +55,15 @@ layoutExpr lexpr@(L _ expr) = do
     HsRecFld{} -> do
       -- TODO
       briDocByExactInlineOnly "HsRecFld" lexpr
-    HsOverLabel{} -> do
-      -- TODO
-      briDocByExactInlineOnly "HsOverLabel{}" lexpr
+#if MIN_VERSION_ghc(8,6,0)   /* ghc-8.6 */
+    HsOverLabel _ext _reboundFromLabel name ->
+#elif MIN_VERSION_ghc(8,2,0) /* ghc-8.2 8.4 */
+    HsOverLabel _reboundFromLabel name ->
+#else                        /* ghc-8.0 */
+    HsOverLabel name ->
+#endif
+      let label = FastString.unpackFS name
+      in docLit . Text.pack $ '#' : label
     HsIPVar{} -> do
       -- TODO
       briDocByExactInlineOnly "HsOverLabel{}" lexpr