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.h

@@ -42,11 +42,14 @@ enum class ExpressionKind {
 enum class Operator {
   Add,
   And,
+  Deref,
   Eq,
+  Mul,
   Neg,
   Not,
   Or,
   Sub,
+  Ptr,
 };
 
 struct Expression;