changelog:
    * moved from map bassed pq to rbt's one in SPF implementation (it seems
    that thereis almost no diff on my graph's bm. mb gonna test it again some
    time in a future:
<map bassed pq>
[tehnerd@rh-dev2 graphs]$ go test  -test.bench="SPF" -test.benchtime=10s
testing: warning: no tests to run
PASS
BenchmarkSPF
  100000     349210 ns/op
ok    algs/graphs 37.856s
[tehnerd@rh-dev2 graphs]$
<rbt bassed pq>
[tehnerd@rh-dev2 graphs]$ go test  -test.bench="SPF" -test.benchtime=10s
testing: warning: no tests to run
PASS
BenchmarkSPF    50000     342550 ns/op
ok    algs/graphs 20.531s
[tehnerd@rh-dev2 graphs]$