This is a port of btoa version 5.2, written by Paul Rutter, Joe
Orost & Stefan Parmark.  btoa converts 4 binary characters to 5
ascii ones, causing a 25% expansion.  (btoa is thus more efficient
than uuencode, which causes a 33% expansion.)  Spaces will not be
used, which should make it safe to send files over e-mail or Usenet
without risking that blanks become tabs.  Each resulting row of
text has a single-byte checksum for error detection.  A diagnosis
file provides a list of errors found this way, which could then be
used to retransmit only the failing lines.