What's with the nothrow_integer type?

Several people on the Boost developers' mailing list expressed reservations about the exception-blocking mechanism in the original implementation, and the Not-a-Number value that it required. After a spirited debate, two indisputable points emerged:

• The existence of a Not-a-Number value forced every function to check its parameters for it.
• The exception-blocking mechanism would cause problems with any library built on this one, such as the arbitrary-precision real type that I'm certain someone will build.

To deal with this, I separated the xint::integer and xint::nothrow_integer types. xint::integer functions now always throw exceptions on errors, and always hold valid numbers; xint::nothrow_integer functions (as implied by the name) never throw exceptions, instead returning the Not-a-Number value (which only nothrow_integer can hold).

This design ensures that the Not-a-Number value only needs to be checked for in the nothrow_integer code, and ensures that any speed penalty from it is only incurred by people who choose to use the nothrow_integer type. It also preserves the no-exceptions behavior, which is useful under some circumstances.