Separated problems from library

Renamed
    problem18and67.py -> constants.py
    library.py -> lib.py
Added problems01to09.py,
    problem10to19.py
    problems20to29.py,
    problems30to39.py,
    problems40to49.py
Removed problem21and39.py

Added problem21 to problems20to29
    problem39 to problems30to39
w.r.t modified lib
renamed:    problem8.txt -> problem08.txt