Add bint() method for rounding towards zero.

Since bfloor() and bceil() exist, add bint() for completeness.  As with
bfloor() and bceil(), bint() does not change the object class. This is
unlike as_int(), which converts the object to a Math::BigInt.

Rename bint() subroutine in the example section to bigint() to avoid
confusion with the new method bint().

Add tests for bint().

Add a little more precise documentation of bfloor() and bceil().