Add 3 '*' operators: one prefix, one infix, and one postfix, per #523. (#582)

The presence or absence of whitespace is used to determine which
operator is in use, following the rules described in #520.

Support for prefix * dereference operator follows #523.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/expression.cpp

@@ -245,15 +245,18 @@ auto Expression::MakeIndex(int line_num, const Expression* exp,
 
 static void PrintOp(Operator op) {
   switch (op) {
-    case Operator::Neg:
-      std::cout << "-";
-      break;
     case Operator::Add:
       std::cout << "+";
       break;
+    case Operator::Neg:
     case Operator::Sub:
       std::cout << "-";
       break;
+    case Operator::Mul:
+    case Operator::Deref:
+    case Operator::Ptr:
+      std::cout << "*";
+      break;
     case Operator::Not:
       std::cout << "not";
       break;