Rename `test::BenchHarness` to `test::Bencher`

`test::BenchHarness` is too long, and not that straightforward. As a type name `Bencher` is more consistent with `#[bench]` attributes.

```
#[cfg(test)]
mod bench {
    use test::Bencher;

    #[bench]
    pub fn foo(b: &mut Bencher) {
    }
}
```
