Cleanup code in itertools::partition
Cleanup code in itertools::partition

Simplify &mut -> &,
fix up parameter names,
and write using match.

stepancheg/rust-itertools,tempbottle/rust-itertools,imp/rust-itertools,flo-l/rust-itertools,Wilfred/rust-itertools,rust-itertools/itertools,DanielKeep/rust-itertools