Fixed big_int_impl

I'm not a friend of this class, because it has nearly 3k lines of code (unmaintable) and is not used currently. There are good libraries out there and they are with good performance.
Also rombust I'd like to ask you to test your code before commiting it, e.g. build one/two examples ;)
How about automatic testing (e.g. all examples, as soon as a new commit has been made?) A jenkins server should do the job.