D.TwoD.Arrow: misc cleanup
D.TwoD.Arrow: simplify definition of arrowBetween'
D.TwoD.Arrow: clarify comments for `arrow` and `arrow'`
change arrow' to produce a delayed leaf.

The idea is that arrow' can now take the *final* transformation into
account so it can properly decide what to draw.  See #112.

Things seem to be almost working... but it seems we don't quite handle
frozen transforms properly yet, since things are flipped in the Y-axis.
D.TwoD.Arrow: bug fix in new arrow': moveTo instead of moveOriginTo
D.TwoD.Arrow: remove debugging output
D.TwoD.Arrow: Use existing line color as default for tail, head, and shaft

This is another cool thing enabled by doing arrows as delayed leaves:
in addition to knowing the overall transformation in effect when
drawing an arrow, we know the overall *style*!  So we can grab
e.g. the line color and use it as a fill color for the arrow head and
tail.  In particular, this patch sets things up so that line color is
used by default for the head, tail, and shaft of an arrow.  That is,
doing (arrow ... # lc blue) will result in a completely blue
arrow (including head and tail).  This can be overridden by setting
headStyle, tailStyle, or shaftStyle as usual.
D.TwoD.Arrow: remove extraneous word in comment
D.TwoD.Arrow: bug fix: actually take unfrozen transform into account!

Previously, arrow' simply took the tranformation and applied it to the
points origin and (len,0), and then drew an arrow between the images
of those points.  However, the transformation might also be doing some
flipping or shearing which was getting ignored: affine transformations
are not determined by their action on two points.

This was causing, e.g. shafts to be drawn flipped along the axis of
the arrow with the SVG backend, since the SVG backend does a `reflectY`
as the last thing before freezing, but it was getting ignored.

This patch makes `arrow'` properly apply the transformation to the
shaft trail prior to uniformly scaling and positioning it.
Merge pull request #137 from diagrams/delayed-subtrees

Delayed subtrees