use `contains` instead of `indexOf` in _.intersection. closes #1417
use _.partial in definition of _.intersection

This takes the tweak suggested in #1417 one step further.
Merge pull request #1418 from davidchambers/intersection

use _.partial in definition of _.intersection
Refactor _.partial.
Cache length.
Merge pull request #1422 from braddunbar/partial-refactor

Refactor _.partial.