Auto merge of #24180 - huonw:optimise-max-etc, r=alexcrichton

Auto merge of #24180 - huonw:optimise-max-etc, r=alexcrichton

The main change in this patch is removing the use of `Option` inside the
inner loops of those functions to avoid comparisons where one branch
will only trigger on the first pass through the loop.

The included benchmarks go from:

    test bench_max    ... bench:       372 ns/iter (+/- 118)
    test bench_max_by ... bench:       428 ns/iter (+/- 33)
    test bench_max_by2 ... bench:      7128 ns/iter (+/- 326)

to:

    test bench_max    ... bench:       317 ns/iter (+/- 64)
    test bench_max_by ... bench:       356 ns/iter (+/- 270)
    test bench_max_by2 ... bench:      1387 ns/iter (+/- 183)

Problem noticed in http://www.reddit.com/r/rust/comments/31syce/using_iterators_to_find_the_index_of_the_min_or/