Add a ".Not()" extension

[martin-oom]

Maybe add a .Not() expression extension.

Instead of doing:

Expression<Func<SomeClass, bool>> myPredicate = x => x.Prop1 == "123";
var negated = Expression.Lambda<Func<SomeClass, bool>>(Expression.Not(myPredicate.Body), myPredicate.Parameters);
// negated = x => !(x.Prop == "123");

one could do:

Expression<Func<SomeClass, bool>> myPredicate = x => x.Prop1 == "123";
var negated = myPredicate.Not();
// negated = x => !(x.Prop == "123");