Remove FunctionDefinition's default constructor (#735)

This opens up a path for switching `int line_num` to a `Location loc`, which I want to do for tracking filenames of code. But I don't think we should have a default constructor on `Location` to avoid mistakes, and switching FunctionDefinition to an arena alloc seems more consistent anyways.

executable_semantics/ast/declaration.h

@@ -72,18 +72,18 @@ class Declaration {
 
 class FunctionDeclaration : public Declaration {
  public:
-  FunctionDeclaration(FunctionDefinition definition)
-      : Declaration(Kind::FunctionDeclaration, definition.line_num),
-        definition(std::move(definition)) {}
+  FunctionDeclaration(const FunctionDefinition* definition)
+      : Declaration(Kind::FunctionDeclaration, definition->line_num),
+        definition(definition) {}
 
   static auto classof(const Declaration* decl) -> bool {
     return decl->Tag() == Kind::FunctionDeclaration;
   }
 
-  auto Definition() const -> const FunctionDefinition& { return definition; }
+  auto Definition() const -> const FunctionDefinition& { return *definition; }
 
  private:
-  FunctionDefinition definition;
+  const FunctionDefinition* definition;
 };
 
 class StructDeclaration : public Declaration {

executable_semantics/ast/function_definition.h

@@ -21,7 +21,6 @@ struct GenericBinding {
 };
 
 struct FunctionDefinition {
-  FunctionDefinition() = default;
   FunctionDefinition(int line_num, std::string name,
                      std::vector<GenericBinding> deduced_params,
                      const TuplePattern* param_pattern,