Move to singular-form names for directories

[benlesh]

After #843 was merged, it made things more ergonomic to use a singular-form directory name for observable types.

I think we should probably move all directories over to singular for for consistency. This way it can be used like:

import {BehaviorSubject} from 'rxjs-es6/subject/BehaviorSubject';
import {immediate} from 'rxjs-es6/scheduler/immediate';
import 'rxjs-es6/operator/map';
import 'rxjs-es6/operator/filter';
// etc.