with Cawlishaw by mail, and he basically confirmed that to these is_*
operations, there's no need to return Decimal(0) and Decimal(1) if
the language supports the False and True booleans.
Also added a few tests for the these functions in extra.decTest, since
they are mostly untested (apart from the doctests).
Thanks Mark Dickinson
fully updated to the latests Decimal Specification (v1.66) and the
latests test cases (v2.56).
Thanks to Mark Dickinson for all his help during this process.