Add a StringFactory for Path which prefixes with type.

Here are our various StringFactory looks:

>>> Vertex(10)
v[10]
>>> Edge(2,Vertex(1),"knows",Vertex(2))
e[2][1-knows->2]
>>> Property("name","marko")
p[name->marko]
>>> VertexProperty(7,"name","marko")
vp[name->marko]
>>> Path([],[Vertex(1),"hello",3])
[v[1], 'hello', 3]

NOTE: this is the same string representation for Gremlin-Java as well.

Given that Path is a core interface, I believe it should have a toString() like the other structure interfaces. I propose:

path[v[1], 'hello', 3]

This will make it easy to distinguish it from a list as well.