Optimize rposition

[dotdash]

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:

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)

[rust-highfive]

r? @aturon

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

@bors: r+ 9a17f62 rollup