Make instances public, add fs2Kafka prefix to their names

[bplommer]

These instances are pretty generic and are useful for clients, especially as some public methods take e.g. NonEmptySet[TopicPartition] which requires an Order instance.

[codecov-io]

Codecov Report

Merging #473 (09bb4cc) into master (35c49bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #473   +/-   ##
=======================================
  Coverage   89.43%   89.43%           
=======================================
  Files          63       63           
  Lines        1467     1467           
  Branches       44       45    +1     
=======================================
  Hits         1312     1312           
  Misses        155      155           

Impacted Files	Coverage Δ
...main/scala/fs2/kafka/CommitRecoveryException.scala	100.00% <ø> (ø)
.../main/scala/fs2/kafka/CommitTimeoutException.scala	100.00% <ø> (ø)
...e/src/main/scala/fs2/kafka/CommittableOffset.scala	100.00% <ø> (ø)
.../main/scala/fs2/kafka/CommittableOffsetBatch.scala	66.66% <ø> (ø)
.../core/src/main/scala/fs2/kafka/KafkaConsumer.scala	98.69% <ø> (ø)
...core/src/main/scala/fs2/kafka/ProducerResult.scala	100.00% <ø> (ø)
...ules/core/src/main/scala/fs2/kafka/instances.scala	100.00% <ø> (ø)
.../scala/fs2/kafka/internal/KafkaConsumerActor.scala	79.85% <ø> (ø)
...e/src/main/scala/fs2/kafka/internal/LogEntry.scala	27.50% <ø> (ø)

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