**minor breaking change**: embed struct rather than name it

This simplifies usage. I'm not quite sure why this was not the original
implementation.

I don't think anybody used this type yet, but the fix is extremely
simple.

pkg/kgo/record_and_fetch.go

@@ -393,8 +393,8 @@ func (fs Fetches) EachPartition(fn func(FetchTopicPartition)) {
  for _, topic := range fetch.Topics {
  for i := range topic.Partitions {
  fn(FetchTopicPartition{
- Topic: topic.Topic,
- Partition: topic.Partitions[i],
+ Topic:  topic.Topic,
+ FetchPartition: topic.Partitions[i],
  })
  }
  }
@@ -447,13 +447,13 @@ func (fs Fetches) EachRecord(fn func(*Record)) {
 type FetchTopicPartition struct {
  // Topic is the topic this is for.
  Topic string
- // Partition is an individual partition within this topic.
- Partition FetchPartition
+ // FetchPartition is an individual partition within this topic.
+ FetchPartition
 }
 
 // EachRecord calls fn for each record in the topic's partition.
 func (r *FetchTopicPartition) EachRecord(fn func(*Record)) {
- for _, r := range r.Partition.Records {
+ for _, r := range r.Records {
  fn(r)
  }
 }