GSL.assert: Assertions

• Expects // precondition assertion. Currently placed in function bodies. Later, should be moved to declarations.

            // `Expects(p)` terminates the program unless `p == true`
            // `Expect` in under control of some options (enforcement, error message, alternatives to terminate)
  

• Ensures // postcondition assertion. Currently placed in function bodies. Later, should be moved to declarations.

These assertions is currently macros (yuck!) pending standard commission decisions on contracts and assertion syntax.