A no-frills converter between 1D and 2D spaces using the Hilbert curve

Most (all?) of the existing CPAN modules state "only works for $foo data",
"optimized for foo situations", or "designed to work as part of the foo
framework". This module is based directly on the example algorithm given on
Wikipedia, and thus is subject only to the single strict limitation of Hilbert
curves: that the side-length 'n' MUST be an integer power of 2.