Update fannkuchredux benchmark

From the discussion on reddit:
http://www.reddit.com/r/rust/comments/2fenlg/benchmark_improvement_fannkuchredux/

This adds two variants: the primary, that uses an unsafe block, and a secondary
that is completely safe.

The one with the unsafe block matches clang's performance and beats gcc's.
Optimize Slice::reverse

This makes the completely safe implementation of fannkuchredux perform
the same as C++. Yay, Rust.
auto merge of #16999 : brson/rust/fannkuch, r=alexcrichton

From the discussion on reddit:
http://www.reddit.com/r/rust/comments/2fenlg/benchmark_improvement_fannkuchredux/

This adds two variants: the primary, that uses an unsafe block, and a secondary
that is completely safe.

The one with the unsafe block matches clang's performance and beats gcc's.