Nagini

Extensive support for information flow verification

* Verification of ordinary non-interference for sequential programs
* Verification of possibilistic and probabilistic non-interference for concurrent programs
* Counterexamples for non-interference proofs

Major updates

* Updated dependency versions, particularly of mypy
* Compatibility with Python 3.8
* Experimental support for counterexamples

Minor changes

* Updated used Viper version to 20.07
* Improved performance for verifying top-level statements and order of declarations
* `nagini_contracts` is marked for mypy as containing type stubs
* Several smaller bug fixes