[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.h

@@ -109,7 +109,7 @@ auto MakeFunType(int line_num, Expression* param, Expression* ret)
     -> Expression*;
 auto MakeAutoType(int line_num) -> Expression*;
 
-void PrintExp(Expression* exp);
+void PrintExp(const Expression* exp);
 
 }  // namespace Carbon