Graph reduce

[dionyziz]

This patch introduces the reduce algorithm which performs a dfs but processes the graph using an arbitrary function accumulator. pre-order and post-order are still supported. DFS has been rewritten using the new reduce function.

Fix #62

[lutzroeder]

@arieljake can you help reviewing and testing this change?

[pkakelas]

LGTM

[mstou]

I did a stress test as we discussed in #94 . It turns out that in both the old and new imlpementations of dfs ( with both 'post' & 'pre' ), a stack overflow is caused at a chain length of around ~2000 nodes(!)

I ran the tests on node v9.2.0.
If you want to replicate the results, the c++ code that generetated the test files is the following:

#include <cstdio>

using namespace std;

const int chainLength = 2000;

int main(){
  printf("var Graph = require('./graphlib').Graph;\n");
  printf("var dfs = require('./graphlib/lib/alg/dfs');\n");
  printf("var g = new Graph();\n");

  for(int i=1; i<=chainLength; i++){
    printf("g.setEdge(%d, %d);\n", i, i+1);
  }

  printf("console.log(%d);\n",chainLength);
  printf("console.log(dfs(g,1,\'post\'))");
}

[dionyziz]

Nice catch, thanks for running this. This indicates a bug in the old implementation as well, so it's a good chance to fix it. I'll work on this.

[dskdas]

I had a similar need and came across this PR while searching around. This is still being reviewed?