Antidivisors

Implemented functions to compute antidivisors and the number of
antidivisors.  Antidivisors are the numbers that do not divide a number
by the largest possible margin.

References
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- [1] http://oeis.org/A066272/a066272a.html
`[3, 5, 15, 17, 51, 85]` - sorted list, isn't?  Perhaps, this is a bad example.