almost have safe multiplication working. Fails 4 test cases. BUT these cases require more than trivial changes so I'm checking in this intermediate version
More carefully defined the concepts. That is the concept of the library is now clear:
Trap at either compile or runtime integer operations which yield a result which differs from the normal arithmetic result.
Pending on this check in:
a) only addition is fixed up so far
b) uses BOOST_TYPEOF... rather than decltype(... - final decision sill pending.
c) comments out some compile time checks which should be re-instated.
d) fixed tests so that they actually work!
