Make UNTIL arity-2, add LOOP-UNTIL & LOOP-WHILE

[hostilefork]

This transitions UNTIL to be arity-2, to be a natural pairing with the
WHILE construct. (In the same spirit of IF vs. UNLESS). The previous
version that is a condition only is then moved to LOOP-UNTIL, as a
way of saying what to do in the missing arity slot of the body. Then
LOOP-WHILE is added to be the arity-1 WHILE complement of that.

Compatibility in <r3-legacy> is accomplished with:

UNTIL: :LOOP-UNTIL

The word LOOP was chosen due to its generic sense. ("What should
I be doing UNTIL that happens...or WHILE it is happening..." => "You
should loop.") That's as opposed to having any specific relationship to
LOOP the construct's parameterization (now or under future redesign).

LOOP-WHILE has slightly more overhead than LOOP-UNTIL, because
it seeks to return the last TRUE? result (instead of being guaranteed to
return the FALSE or NONE value that caused the loop to break). Thus
LOOP-UNTIL remains the "most efficient" base looping construct.

[hostilefork]

I'll mention that originally the rebol-proposals version of WHILE and UNTIL had a refinement /AFTER. It instructed the condition to be run after the loop body instead of before. That was somewhat unnatural looking, and LOOP-UNTIL and LOOP-WHILE offer a clear enough alternative.

So /AFTER is omitted from this.

[hostilefork]

In consideration of the established people who are interested in this language, the ostensible linguistic correctness of this is eclipsed by their investment.

Your pull request is therefore closed.