RecordBatchFileReader and RecordBatchStreamReader are
interfaces for generating record batches from different input sources.
The RecordBatchFileReader$create() and RecordBatchStreamReader$create()
factory methods instantiate the object and
take a single argument, named according to the class:
file A character file name, raw vector, or Arrow file connection object
(e.g. RandomAccessFile).
stream A raw vector, Buffer, or InputStream.
$read_next_batch(): Returns a RecordBatch
$schema(): Returns a Schema
$batches(): Returns a list of RecordBatches
$get_batch(i): For RecordBatchFileReader, return a particular batch
by an integer index.
$num_record_batches(): For RecordBatchFileReader, see how many batches
are in the file.