Store named constant values in the AST (#1011)

This enables us to stop using `Env` in the typechecker. As a byproduct, this commit also restructures the interpreter to handle run-time global initialization as part of ordinary execution, using the Action stack.

executable_semantics/ast/statement.h

@@ -344,6 +344,9 @@ class Continuation : public Statement {
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
   auto value_category() const -> ValueCategory { return ValueCategory::Var; }
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
 
  private:
   std::string name_;