Implementing += operator for record types.

This is per #375.

Record types can now get additional fields later via '+='. The added
fields must however either be &optional or have a &default value.

Example:

    type Foo: record {
        a: count;
        b: count &optional;
    };

    redef record Foo += {
        c: count &default=42;
        d: count &optional;
    };

    global f: Foo = [$a=21];

    print f;

Output:

    [a=21, b=<uninitialized>, c=42, d=<uninitialized>]

src/Type.cc

@@ -898,6 +898,26 @@ void RecordType::Describe(ODesc* d) const
 		}
 	}
 
+const char* RecordType::AddFields(type_decl_list* others)
+	{
+	assert(types);
+
+	loop_over_list(*others, i)
+		{
+		TypeDecl* td = (*others)[i];
+
+		if ( ! td->FindAttr(ATTR_DEFAULT) && ! td->FindAttr(ATTR_OPTIONAL) )
+			return "extension field must be &optional or have &default";
+
+		types->append(td);
+		}
+
+	delete others;
+
+	num_fields = types->length();
+	return 0;
+	}
+
 void RecordType::DescribeFields(ODesc* d) const
 	{
 	if ( d->IsReadable() )

src/Type.h

@@ -409,6 +409,10 @@ public:
 
 	int NumFields() const			{ return num_fields; }
 
+	// Returns 0 if all is ok, otherwise a pointer to an error message. Takes
+	// ownership of list.
+	const char* AddFields(type_decl_list* types);
+
 	void Describe(ODesc* d) const;
 	void DescribeFields(ODesc* d) const;
 

src/parse.y

@@ -815,6 +815,24 @@ decl:
 				}
 			}
 
+	|	TOK_REDEF TOK_RECORD global_id TOK_ADD_TO
+			'{' type_decl_list '}' ';'
+			{
+			if ( ! $3->Type() )
+				$3->Error("unknown identifier");
+			else
+				{
+				RecordType* add_to = $3->Type()->AsRecordType();
+				if ( ! add_to )
+					$3->Error("not a record type");
+				else {
+					const char* error = add_to->AddFields($6);
+					if ( error )
+    					$3->Error(error);
+				}
+			}
+    	}
+
 	|	TOK_TYPE global_id ':' refined_type opt_attr ';'
 			{
 			add_type($2, $4, $5, 0);