Perl ordered hash class implementation

 Hash::Ordered implements an ordered hash, meaning that it associates keys
 with values like a Perl hash, but keeps the keys in a consistent order.
 Because it is implemented as an object and manipulated with method calls, it
 is much slower than a Perl hash. This is the cost of keeping order.
 .
 However, compared to other ordered hash implementations, Hash::Ordered is
 optimized for getting and setting individual elements and is generally faster
 at most other tasks as well. For specific details, see
 Hash::Ordered::Benchmarks.