AnyRecordBatch: enum over data interface and stream interface.

[kylebarron]

Used for function input when a function can take either one or a stream of record batches.

This was originally named AnyArray, but I think separating AnyArray and AnyRecordBatch makes more sense. AnyRecordBatch is an enum over RecordBatch and Stream, where you get one or more RecordBatch objects. A future AnyArray would be an enum over Array and Stream, but that would use an ArrayReader, similar to the current RecordBatchReader but where it doesn't unpack the struct array.