another proposed fix for Seaside31 Issue glassdb/Seaside31#21

- this time asString interprets the bytes in the receiver as it's
  face value ... even for Utf8 instances ...
- the code is pretty funky, because all of the normal ways of
  converting a ByteArray into a String (i.e., String
  class>>withBytes:) are not permitted with instances of Utf8, so
  I had to resort to #changeClassTo: which is not allowed to be
  sent to an object on the stack ... so a copy is made ...
- with this checkin ByteArray>>asString has the implementation for
  UTf8. A pure ByteArray can use String class>>withBytes: without
  the  #changeClassTo:, but I want to make sure that this trick
  works for all 3.x versions and that all of the tests pass ...