Contracts and Harnesses for <*const T>::add, sub and offset (rust-lang#166)

Towards rust-lang#76 

**Summary**
* Adds contracts for <*const T>::add, sub and offset.
* Adds proof for contracts for above methods, verifying following
pointee types:
    * All integer types
    * Tuples (representing composite types)
    * Slices
    * Unit type

Author: xsxszab