optimize number field arithmetic using flint and singular

[spancratz]

C code for rational polynomials based on FLINT1

[was]

basic refactoring of number fields elements -- only arithmetic with absolute fields works now.

[spancratz]

Including basic support for flint

[was]

flattened of everything. apply only this.

[was]

I will start posting part1, part2, etc. Then when all is done, a flattened version of it all.