Numstat / fix interface

Further number/coeffs interface fixes (part2)

fix: correct header includes 
chg: minor cleanup
note: dangerous handing of BIGINTs was marked
@malex984 
@hannes14 

the mark
//BUG: currRing->cf);?
is not worth a penny. 
Same as Gerhards '//HIER' in 'primdec.lib'

Please open a ticket
either here
https://github.com/Singular/Sources/issues
or here
http://www.singular.uni-kl.de:8002/trac/

Jakob