Threads should eventually take and produce $cage instead of $vase. Since
%khan is likely to be used by third parties, we write to the eventual
intended API. We ignore the mark on the input $cage (it is safe to
always specify %noun), and we always use %noun as the output mark.
%fyrd now makes more sense. It was previously discarding the type of the
output %arow and re-encoding the raw noun as a vase of the output mark;
it is now performing mark conversion from the mark of the output $cage
to the originally requested output mark.
Also strips out `$` from khan top-level comment.
There are arguments for keeping $crag in lull, and on the other side for
moving $cast to arvo. This seemed like the most reasonable approach.
%fyrd is now implemented in terms of %fard, and likewise %avow in terms
of %arow. State is tracked via wire rather than in a global map.
Unit tests adjusted to match.
;;(vase ...) does a nest-check of the type of the kernel. This is
undesirable, so we instead run everything through +slum and cast the
result to +tang.
