remove errant part of SFV.inspect
implement SFV#merge
make SFV#merge non-destructive

add an extra test case to make sure this gets caught
allow specification of a vector when creating a leaf
rough in {InternalNode,Leaf}#search
make InternalNode#new take an initial set of children
correct order calculation for internal nodes and leaf nodes
self-encapsulate vector_length in SFV
define #full? on leaf, internal node
tweak #inspect, define #to_s on SFV
spec and implement #[] on SFV
tweak InternalNode#inspect