Rewrite imports in es6 folder

[OliverJAsh]

Currently the imports in the es6 folder reference fp-ts/lib when they should reference fp-ts/es6 (so that tree shaking can work properly), e.g.

https://unpkg.com/browse/@devexperts/remote-data-ts@2.0.3/es6/remote-data.js

import { constFalse, identity, constant } from 'fp-ts/lib/function';

This change has recently been made to other libraries such as io-ts and fp-ts-rxjs, for example: gcanti/fp-ts-rxjs@f35c926

https://unpkg.com/browse/fp-ts-rxjs@0.6.6/es6/Observable.js

Related issue: gcanti/fp-ts#1044