Added looping ground work.
Mainly the loopStack in D4ScriptEngine.
Added initial stack operators (drop, dup, swap etc.).
Added associated unit test method.
Added loop-until control flow construct.
Added associated unit test method.
Changed variable assignment order (i.e. the set core word parameter
order).
Makes it simpler to set a variable with a value already on the stack
(removing the need for a swap).