[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/interpreter/value.cpp

@@ -197,13 +197,14 @@ auto MakeVoidTypeVal() -> Value* {
   return v;
 }
 
-auto MakeChoiceTypeVal(std::string* name,
+auto MakeChoiceTypeVal(std::string name,
                        std::list<std::pair<std::string, Value*>>* alts)
     -> Value* {
   auto* v = new Value();
   v->alive = true;
   v->tag = ValKind::ChoiceTV;
-  v->u.choice_type.name = name;
+  // Transitional leak: when we get rid of all pointers, this will disappear.
+  v->u.choice_type.name = new std::string(name);
   v->u.choice_type.alternatives = alts;
   return v;
 }