shootout-fannkuch-redux rewrite

Less bound checking and parallelisation.  Brute speed improvement
is about 15% faster.
auto merge of #13633 : TeXitoi/rust/shootout-fannkuch-redux-rewrite, r=alexcrichton

Less bound checking and parallelisation.  Brute speed improvement
is about 15% faster.

The unsafe block improve the brute speed by about 5%.