Contracts and Harnesses for <*mut T>::add, sub and offset

[stogaru]

Towards #76

Changes

• Adds contracts for <*mut T>::add, <*mut T>::sub and <*mut T>::offset
• Adds proofs for contracts of the above functions verifying the pointee types:
  • All integer types
  • Tuples (composite type)
  • Unit Type
• Defines a macro for add and sub and another for offset.

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