Change Alternative to a class (#853)

Revives BisonWrap because this seems a reasonable use of it (avoiding the need to have an std::optional or pointer for Alternative, both of which I thought could be unclear about the intent).

executable_semantics/ast/declaration.cpp

@@ -30,8 +30,8 @@ void Declaration::Print(llvm::raw_ostream& out) const {
     case Kind::ChoiceDeclaration: {
       const auto& choice = cast<ChoiceDeclaration>(*this);
       out << "choice " << choice.Name() << " {\n";
-      for (const auto& [name, signature] : choice.Alternatives()) {
-        out << "alt " << name << " " << *signature << ";\n";
+      for (const auto& alt : choice.Alternatives()) {
+        out << "alt " << alt.name() << " " << alt.signature() << ";\n";
       }
       out << "}\n";
       break;

executable_semantics/ast/declaration.h

@@ -95,10 +95,21 @@ class ClassDeclaration : public Declaration {
 
 class ChoiceDeclaration : public Declaration {
  public:
-  ChoiceDeclaration(
-      SourceLocation loc, std::string name,
-      std::vector<std::pair<std::string, Nonnull<const Expression*>>>
-          alternatives)
+  class Alternative {
+   public:
+    Alternative(std::string name, Nonnull<const Expression*> signature)
+        : name_(name), signature_(signature) {}
+
+    auto name() const -> const std::string& { return name_; }
+    auto signature() const -> const Expression& { return *signature_; }
+
+   private:
+    std::string name_;
+    Nonnull<const Expression*> signature_;
+  };
+
+  ChoiceDeclaration(SourceLocation loc, std::string name,
+                    std::vector<Alternative> alternatives)
       : Declaration(Kind::ChoiceDeclaration, loc),
         name(std::move(name)),
         alternatives(std::move(alternatives)) {}
@@ -108,14 +119,13 @@ class ChoiceDeclaration : public Declaration {
   }
 
   auto Name() const -> const std::string& { return name; }
-  auto Alternatives() const -> const
-      std::vector<std::pair<std::string, Nonnull<const Expression*>>>& {
+  auto Alternatives() const -> const std::vector<Alternative>& {
     return alternatives;
   }
 
  private:
   std::string name;
-  std::vector<std::pair<std::string, Nonnull<const Expression*>>> alternatives;
+  std::vector<Alternative> alternatives;
 };
 
 // Global variable definition implements the Declaration concept.