A library to get stack trace from C++ exceptions.

This is proof of concept code, if you are interested, feel free to improve. It 
is annoying, that one cannot get stack trace from exceptions in C++, so this 
code should allow for this.

This is by no means production code, but as it stands it seems to work, at 
least with my non-automated tests.

Tried a few different approaches and the current one seems to work. I would 
have liked not to need any info about exceptions in the module, keep all info 
in exception allocated memory itself. Unfortunatelly because C++ library 
allocates extra memory below the exception address and I did not find a way how 
to determine the size of it, need to keep the extra memory for stacktrace above
the exception, so one needs to remember size of the exception itself to be able
to get to the stack trace block. Other option would be to keep whole stack trace
in its own memory, but that did not feel as the right approach.

I have looked around, but unfortunatelly did not find any library, which would 
just work, without any additional instrumentation. Also wanted that stack trace 
is available for exceptions thrown from standard or third party libraries. 

Here is a list of libraries I had a look at:
* libunwind (http://www.nongnu.org/libunwind/)
* libcsdbg (https://sourceforge.net/projects/libcsdbg)
* Poppy (https://github.com/orling/Poppy)
* ex_diag (https://code.google.com/archive/p/exception-diagnostic/)
* c-callstack (https://github.com/Nanolat/c-callstack)
* backward-cpp (https://github.com/bombela/backward-cpp)
* dbg library (https://bitbucket.org/edd/dbg/wiki/exception_tracebacks)

Vladimir Michl