Change loom.alg-generic/pre-traverse to do much less work

[jafingerhut]

especially if the graph is dense with edges for a node that is
traversed, but the returned lazy sequence is not ever evaluated to get
to those nodes.

Also eliminate a stack-growing recursive call, using recur instead.
The previous version could grow the call stack nearly as deep as the
number of edges in the graph.

[jafingerhut]

I was using pre-traverse to do a partial DFS of some large graphs where some nodes had very high degree, and found that it was doing a lot of unnecessary work for edges in the graph that were never 'crossed' by my partial DFS.