adds Len for fast element retrieval
adds benchmark against a map[string]struct{}

on my on my i5-2400:
BenchmarkMap	   10000	    105894 ns/op	    5085 B/op	      10 allocs/op
BenchmarkTree	  10000	    289837 ns/op	    5315 B/op	     162 allocs/op

looks not great dropping the interface{} could help
boost performance by a third by replacing the use of interface{} with struct{string; *node}

on my on my i5-2400:
BenchmarkMap	   10000	    103756 ns/op	    5064 B/op	      10 allocs/op
BenchmarkTree	  10000	    175455 ns/op	    5192 B/op	      81 allocs/op

not awesome but definitely much better