add envelopes to monoidal measures on segments
trails are now finger trees of segments.
Remove the Functor instance for Trail

Trail actually can't be a Functor anymore, because of the caching of
monoidal annotations.  Given an arbitrary function (v1 -> v2) we have
no idea how to update the arc length and so on for the new vector
space.  Actually this seems to imply that Trail never should have been
a functor in the first place!  Or perhaps it is a functor, but in some
category other than Hask (i.e. in the category of vector spaces and
linear transformations).  We were just getting away with it
because of the extremely simple representation.