Making way for checked_float, checked_money, etc
a) factoring checked into checked_integer etc.
b) moved some tests from test_z to official tests
c) added test_rational which tests interoperability with boost::rational
d) moved safe_range to safe_integer_range
before this change, it would get instantiated by is_convertible<T, safe<U>> for any type T which created a syntax error.
This problem appears with using rational<safe<int>> but passes tests now.
Switched casting operators back to implicit. Having it explicit makes it safe types more unwieldy to use and breaks some examples.
passes all tests
adds documentation of library internals
implements trap_exception for compile time guarantee for program correctness
still needs update to support the above for operations in addition to + and -
This lead to numerous small changes - which of course weren't so small
Added static asserts for some range construction / assignments.
Passing all tests.
