datastore/query.go
func DecodeCursor(s string) (Cursor, error) {
	if s == "" {
		return Cursor{&zeroCC}, nil
	}
	if n := len(s) % 4; n != 0 {
		s += strings.Repeat("=", 4-n)
	}
	b, err := base64.URLEncoding.DecodeString(s)
	if err != nil {
		return Cursor{}, err
	}
	cc := &pb.CompiledCursor{}
	if err := proto.Unmarshal(b, cc); err != nil {
		return Cursor{}, err
	}
	return Cursor{cc}, nil
}
func DecodeCursor(s string) (Cursor, error) {
	if s == "" {
		return Cursor{&zeroCC}, nil
	}
	if n := len(s) % 4; n != 0 {
		s += strings.Repeat("=", 4-n)
	}
	b, err := base64.URLEncoding.DecodeString(s)
	if err != nil {
		return Cursor{}, err
	}
	cc := &pb.CompiledCursor{}
	if err := proto.Unmarshal(b, cc); err != nil {
		return Cursor{}, err
	}
	return Cursor{cc}, nil
}
// Decode decodes a cursor from its base-64 string representation.