Optimize rposition

The extra check caused by the expect() call can, in general, not be
optimized away, because the length of the iterator is unknown at compile
time, causing a noticable slow-down. Since the check only triggers if
the element isn't actually found in the iterator, i.e. it isn't
guaranteed to trigger for ill-behaved ExactSizeIterators, it seems
reasonable to switch to an implementation that doesn't need the check
and just always returns None if the value isn't found.

Benchmark:
````rust
let v: Vec<u8> = (0..1024*65).map(|_| 0).collect();
b.iter(|| {
    v.as_slice().iter().rposition(|&c| c == 1)
});
````

Before:
````
test rposition  ... bench:     49939 ns/iter (+/- 23)
````

After:
````
test rposition  ... bench:     33306 ns/iter (+/- 68)
````

0 files changed, 0 insertions, 0 deletions