ndarray-parallel crate: Rayon + ndarray integration, as a crate in the land between

[bluss]

• new crate ndarray-parallel that pulls in both rayon and ndarray
• This is strategy (2) (See https://github.com/nikomatsakis/rayon/pull/188#issuecomment-269633834)
• This is a separate crate until rayon clarifies its version story somewhat.