Implements a S4 hash class in R similar to hashes / associatesd arrays /
dictionaries in other programming languages. Where possible, the hash
class uses the standard R accessors: $, [ and [[.
Hash construction is flexible and takes several syntaxes and all hash
operations are supported.
For shorter key-value pairs, lists might yield higher performance, but for
lists of appreciable length hash objects handly outperform native lists.