Add benchmarks for filter, map and reduce

Running the included benchmarks with `go test -bench .` yields the following results on my machine:

```
BenchmarkFilter	  200000	     41188 ns/op
BenchmarkNoFilter	 5000000	       640 ns/op
BenchmarkMap	   10000	    108577 ns/op
BenchmarkNoMap	10000000	       321 ns/op
BenchmarkReduce	 1000000	      1962 ns/op
BenchmarkNoReduce	100000000	        22.0 ns/op
```

Which means: 

Filter: 64x slower
Map: 338x slower
Reduce: 89x slower

I like the convenience this package gives, but I think it should be transparent how much slower this approach is, in order to make the right decision.