[executable semantics] class-ify Declaration (#307)

* [executable semantics] class-ify Declaration

NFC (no functional change).

Proof of concept that we can simplify code by replacing unions with safer, more
regular types.  Hand-rolled existentials (type-erasing CoW wrappers) are a follow-on
step that will further simplify usage.

Began adding `const` where possible, and replacing `std::string*` with
`std::string`.  Most `const`s can disappear as we replace reference semantics
with value semantics, but in the meantime it's an important step in the right
direction.

executable_semantics/ast/expression.cpp

@@ -197,7 +197,7 @@ static void PrintFields(
   }
 }
 
-void PrintExp(Expression* e) {
+void PrintExp(const Expression* e) {
   switch (e->tag) {
     case ExpressionKind::Index:
       PrintExp(e->u.index.aggregate);