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/interpreter/value.cpp

@@ -357,9 +357,8 @@ auto PrintValue(const Value* val, std::ostream& out) -> void {
       out << "Continuation";
       break;
     case ValKind::PointerTV:
-      out << "Ptr(";
       PrintValue(val->GetPointerType().type, out);
-      out << ")";
+      out << "*";
       break;
     case ValKind::FunctionTV:
       out << "fn ";