Migrate Declaration to variant (#644)

Note this makes some structural choices that I'm not sure how popular they'll be... Most notably, I could remove `DeclarationKind` because I'm using `std::visit` and `Declaration::Visitor` in reality. I could go to `switch(tag())` instead, but I'm wondering if this approach will be well received for the separation of ownership. Alternatively, I could move `Declaration` around so that it actually implements the things like `TopLevel` -- I did feel weird with the old structure of typecheck.cpp implementing members of declaration.h, though.

executable_semantics/ast/struct_definition.h

@@ -14,8 +14,8 @@ namespace Carbon {
 
 struct StructDefinition {
   int line_num;
-  std::string* name;
-  std::list<Member*>* members;
+  std::string name;
+  std::list<Member*> members;
 };
 
 }  // namespace Carbon