Added initial hash core words (@set @put etc.).
Added associated unit test method.
Fixed access level of some internal methods.
Fixed exception metadata handling in D4ScriptEngine.evalScript().
Added D4Var.Hash.keys() and D4Var.Hash.values().
Added associated unit test method.
Added @keys & @values hash operations.
Added associated unit test method (preliminary).
Added size stack operator.
Added associated test code in unit test.
Added qw core word.
Added associated unit test method.
Added D4Var.undef() and D4Var.isUndef().
Added associated unit test method.
Added undef and isundef core words.
Added associated unit test method.