With Python, the whole Integer vs. BigInteger is slightly useless

- for the moment lets use sys.maxint as a limit, will see whether we need to use the more traditional 31-bit boundaries for compatibility

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Adapted BigInteger primitives to use Integer.value_fits

- also removed redundant checks and introduced polymorphic getter for the embedded value in BigInteger and Integer
- used biginteger spelling for field name

Signed-off-by: Stefan Marr <git@stefan-marr.de>