Feature request: Add tracing support, for debugging purposes

[adfoster-r7]

It would be cool to add support for outputting parsing information as the input stream is context. This functionality would have been very useful at the start when I was implementing the initial Kerberos specification

As an example - a library that does this is https://github.com/dmendel/bindata which is a declarative library for parsing binary structure which has a tracing feature:

require 'bindata'

class Polygon < BinData::Record
  endian :little
  uint8 :num_points, :value => lambda { points.length }
  array :points, :initial_length => :num_points do
    double :x
    double :y
  end
end

data =
  "\x03\x00\x00\x00\x00\x00\x00\xf0\x3f\x00\x00\x00\x00\x00\x00\x00" \
  "\x40\x00\x00\x00\x00\x00\x00\x08\x40\x00\x00\x00\x00\x00\x00\x10" \
  "\x40\x00\x00\x00\x00\x00\x00\x14\x40\x00\x00\x00\x00\x00\x00\x18" \
  "\x40".b

BinData::trace_reading($stdout) do
  puts Polygon.read(data)
end

Outputs:

obj.num_points => 3
obj.points[0].x => 1.0
obj.points[0].y => 2.0
obj.points[1].x => 3.0
obj.points[1].y => 4.0
obj.points[2].x => 5.0
obj.points[2].y => 6.0
{:num_points=>3, :points=>[{:x=>1.0, :y=>2.0}, {:x=>3.0, :y=>4.0}, {:x=>5.0, :y=>6.0}]}

The API could be similar for RASN1, and it would be useful for users that are parsing complex structures. At the minute a parsing failure results in a crash, and I manually delete fields until I get things to parse again successfully