Standard compliance and more robust testing

• Removing double underscores
• Unit-Tests work in release mode as well

explicitely enable and disable contracts

• Compiler flag BERTRAND_ENABLE_CONTRACTS and BERTRAND_DISABLE_CONTRACTS for forced enabled/diasabled contracts.
• Cmake linting

Linting and Standard Compliance

• Overall linting of code
• Removing double underscores from macros for compliance with C++ Standards
• Contract keywords start with capital letters to avoid confusion with C++20 concepts

Compatibility with MSCV

• Fixed compilation failure in release mode with in MSVC
• When defined NDEBUG no code is generated

Expression in contract message

Merge pull request #10 from bernedom/feature/expression-in-message

Added expression to message

Mandatory message for contracts

Contracts require (pun intended) a message together with the boolean expression.
Also bertrand no longer uses for implementation but directly aborts()

Initial release

Initial release - trivial implementation