Fix warnings reporting and stdlib warnings (#367)

* Fix stdlib warnings

* Mark unused includes when used from non-included files

* Do not mark indirectly included files as unused

* Show unused include warning only for files that are never used

* Remove unused include from Option.aes

* Consider functions passed as args as used

* Return warnings as a sorted list

* Fix failing tests

* Fix dialyzer warning

* Fix warning in Func.aes

priv/stdlib/ListInternal.aes

@@ -2,8 +2,8 @@ namespace ListInternal =
 
   // -- Flatmap ----------------------------------------------------------------
 
-  function flat_map(f : 'a => list('b), xs : list('a)) : list('b) =
-    switch(xs)
+  function flat_map(f : 'a => list('b), lst : list('a)) : list('b) =
+    switch(lst)
       []      => []
       x :: xs => f(x) ++ flat_map(f, xs)