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

test/contracts/manual_stdlib_include.aes

@@ -1,4 +1,4 @@
-// This should include Lists.aes implicitly, since Option.aes does.
+include "List.aes"
 include "Option.aes"
 
 contract Test =

test/contracts/warnings.aes

@@ -47,3 +47,14 @@ contract Warnings =
   entrypoint unused_return_value() =
     rv()
     2
+
+namespace FunctionsAsArgs =
+    function f() = g()
+
+    private function g() = h(inc)
+    private function h(fn : (int => int)) = fn(1)
+
+    // Passed as arg to h in g
+    private function inc(n : int) : int = n + 1
+    // Never used
+    private function dec(n : int) : int = n - 1