GSoC 2016: Updated stack trace format

[ghost]

This PR:

• Introduces a new format for the stack trace
• Provides a system-wide prediff for the testing system that removes the stack trace from some specific test output.

[ghost]

As an example of the new format, this program

proc main(){
  fact(2);
}

proc fact(i : int) :int{
  if(i == 0) then halt();
  else return i*fact(i-1);
}

produces

test.chpl:6: error: halt reached
Stacktrace

halt() at $CHPL_HOME/modules/internal/ChapelIO.chpl:679
fact() at prova.chpl:5
fact() at prova.chpl:5
fact() at prova.chpl:5

[mppf]

Is the logic above really necessary? Also, I'd prefer this prediff be written in Python or Perl. It would be better to read the output one line at a time and check for the Stacktrace region. As it is, I think the grepping below might mess up test output. But there's no real reason to change anything at all until we get to a region just after a line that was exclusively "Stacktrace".