Pointers (#1060)

* Naive pointer implementation

* Bug fixes and better pointer tests

* Remove debug print statement

* Implement changes suggested by @geoffromer

Also add a failing test case that tests applying the address-of operator
to an rvalue.

executable_semantics/ast/expression.h

@@ -96,6 +96,7 @@ class FieldInitializer {
 
 enum class Operator {
   Add,
+  AddressOf,
   And,
   Deref,
   Eq,