Allow binary operators to be used as lambdas (#385)

* Add operator lambdas

* Do not register anonymous functions as called functions

* Add tests

* Update CHANGELOG

* Update the docs

* Do not allow (..) to be used as a lambda

* Rename the function sum to any

test/contracts/operator_lambdas.aes

@@ -0,0 +1,14 @@
+include "List.aes"
+
+contract C =
+    type state = int
+
+    function any(l : list(bool)) : bool = List.foldl((||), false, l)
+
+    entrypoint init() =
+        let bad_application = (+)(1)
+        let good_application = (-)(3, 4)
+        let op_var = (+)
+        let op_var_application = op_var(3, 4)
+
+        good_application + op_var_application