Work in progress - very ham-fisted cutover that introduces the notion of a
unique path that acts as a prefix to any node in the ternary search tree where
branching takes place. The unique path is compressed down into a list associated
with the node. The logic is definitely erroneous in places, but the prefixes are
not yet propulated - the point of this is to cutover the type signatures to see
where the real work needs doing.

--HG--
branch : PathCompressionWhenMergingTestVectors