GTFS enums usually fit into one byte, so we don't need an int. GTFS
tables usually less than 16 fields, so we need a byte or a short int for
storing a bitmask of assigned fields.

Smaller fields allow us to save memory for the largest table -
stop_times.txt that may have 30 M lines.

Instead of:

class GtfsStopTime {
  // ...
  private int pickupType;
  private int dropOffType;
  private int continuousPickup;
  private int continuousDropOff;
  private int timepoint;
  private int bitField0_;
}

we have now:

class GtfsStopTime {
  // ...
  private byte pickupType;
  private byte dropOffType;
  private byte continuousPickup;
  private byte continuousDropOff;
  private byte timepoint;
  private short bitField0_;
}

which is 5 * (4 - 1) + 2 = 17 bytes smaller. Java aligns classes
by 8 bytes, so we actually save 16 bytes per line.

Total save: 0.5 GiB for 30 M lines in stop_times.txt