Implemented "Fixnum#** overflows the answer to a bignum transparantly"-spec

Stuff like 2**63, where the base and the exponent are not big but the result is, works now.

The following problem occured while untagging:
The "Fixnum#** switches to a Float when the number is too big"-spec started looping forever. I had to force it to fail before it could start looping so that my "invoke specs.untag" call could terminate (the option "-e overflows" for just selecting the spec I was interested in worked for specs.run but not for specs.untag). I'm assuming the problem comes from the pow method of rbigint (in rpython.rlib) but I'm not entirely sure.