Partial support for Integral Classes.

Added a simple Integral Class to Classes supporting:

+ div
+ mod

toInteger and other suitable methods need to be decided upon, defined, and implemented.

Instances:

+ Nat uses total functiond
+ Int, Integer, Bits8-64 are partial.
Merge pull request #680 from jfdm/prelude/integrals

Partial support for Integrals.