Factor out a Pattern sum type from Expression (#685)

`Pattern` is intended to pilot some changes I would like to apply to all our sum types:
- The alternatives are expressed as derived classes rather than members of a `std::variant`.
- The alternatives are classes in the [style guide sense](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes), meaning they can have invariants, but can't have public data members.
- Creating an object is expressed using a constructor rather than a factory function.
- Accessing an alternative is expressed as a cast (using LLVM's RTTI system) rather than `std::get` or a `Get` method.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/declaration.cpp

@@ -35,14 +35,13 @@ auto Declaration::MakeChoiceDeclaration(
   return d;
 }
 
-auto Declaration::MakeVariableDeclaration(int source_location, std::string name,
-                                          const Expression* type,
+auto Declaration::MakeVariableDeclaration(int source_location,
+                                          const BindingPattern* binding,
                                           const Expression* initializer)
     -> const Declaration {
   Declaration d;
   d.value = VariableDeclaration({.source_location = source_location,
-                                 .name = std::move(name),
-                                 .type = type,
+                                 .binding = binding,
                                  .initializer = initializer});
   return d;
 }
@@ -91,8 +90,7 @@ void Declaration::Print(llvm::raw_ostream& out) const {
 
     case DeclarationKind::VariableDeclaration: {
       const auto& var = GetVariableDeclaration();
-      out << "var " << *var.type << " : " << var.name << " = "
-          << *var.initializer << "\n";
+      out << "var " << *var.binding << " = " << *var.initializer << "\n";
       break;
     }
   }