#15 Public debug interface

[cchris]

For various purposes, the contents of all availeble
variables in a running program could be accessible to a
third party process (contents of Eu's ex.err, globally).
For instance, this may allow to to execute(external text
referring to program's variables).
Coupled with some error signalling scheme, this might
allow for a feature-not-so-rich interpreter with ability for
external plug-ins to handle the errors in a variety of
ways, with the added benefit of allowing more people to
contribute to the project, since writing a plug-in is
supposedly easier than writing the interpreter!.