effort isn't used, so report should keep the effort = 1

The `effort` argument to bench & friends is completely ignored during the actual timing, but it is used when printing out a `Benched` instance.  This is really misleading -- cranking up `effort` keeps bringing down the runtimes.  Eg. this:

```
    th.pbench(1 + 2, effort = 1)
    th.pbench(2 + 1, effort=1000)
```

leads to:

```
Benchmark (1342177260 calls in 2.559 s)
  Time:    1.892 ns   95% CI 1.827 ns - 1.957 ns   (n=20)
Benchmark (1342177260 calls in 2.645 s)
  Time:    0.001961 ns   95% CI 0.001898 ns - 0.002024 ns   (n=20)
```

This PR isn't really doing the right thing, which would be to use `effort` correctly.  I'm not sure how you mean for it to be used, so instead I am just always setting it to 1 in `Benched`.