Support ByteArray with UnliftedFFITypes

Motivation

I have a bunch of places in library where I pass a ByteArray to a C function through the FFI. For example:

foreign import ccall unsafe "rd_kafka_topic_partition_list_add"
  rdKafkaTopicPartitionListAdd ::
       Ptr TopicPartitionList                                                                  
    -> ByteArray#
    -> Partition
    -> IO (Ptr TopicPartition)

Every time I do this, I have to unwrap the ByteArray to expose the ByteArray# that I am trying to get at.

Proposal

When UnliftedFFITypes is enabled, allow ByteArray as an argument type in FFI calls. This is possible now that ByteArray has been moved to base.

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