Add benchmark to compare Iter.N to normal for loop

This adds benchmark code to compare the performance of `Iter.N` to an
init-condition-post for loop.

Based on this benchmark it looks like `Iter.N` has some overhead for very
small loops, but quickly becomes almost negligible for large loop values.

This is the output on my machine (Core i5-3470 3.20GHz, Windows 7 64-bit):
```
>go test -bench=.
PASS
BenchmarkIter1e0        100000000               13.7 ns/op
BenchmarkIter1e1        50000000                21.6 ns/op
BenchmarkIter1e2        20000000                84.6 ns/op
BenchmarkIter1e3         3000000               607 ns/op
BenchmarkIter1e4          200000              5860 ns/op
BenchmarkIter1e5           20000             60156 ns/op
BenchmarkIter1e6            2000            612561 ns/op
BenchmarkFor01e0        2000000000               0.92 ns/op
BenchmarkFor01e1        300000000                6.02 ns/op
BenchmarkFor01e2        20000000                62.2 ns/op
BenchmarkFor01e3         3000000               547 ns/op
BenchmarkFor01e4          300000              5703 ns/op
BenchmarkFor01e5           30000             58439 ns/op
BenchmarkFor01e6            2000            581558 ns/op
ok      github.com/bradfitz/iter        24.262s
```

In these results small loops incur a relatively large overhead
(e.g. n=10 takes 3.6x as long), but for larger loops the relative overhead
becomes much smaller (e.g. n=1 million takes 1.05x as long).