Expose fp-ts modules without lib/es6 prefix

[StefanoMagrassi]

This PR allows importing fp-ts modules without specifing the /lib or /es6 prefix:

// from
import * as O from 'fp-ts/lib/Option'

// to
import * as O from 'fp-ts/Option'

Build tools will automatically resolve the right path based on the current environment, pointing to /lib version for commonjs and /es6 version for ES6.

For now the /lib and es6 paths are still available, so this feature is completely backward compatible.

This should enhance the library ergonomics and tree-shakeability.

Note: in order to achieve this goal, the standard npm publish command was disabled in favour of an npm run release command which builds the library and prepares the package for publication.

ref #1227 #1053

[waynevanson]

I'm extremely eager for this! Great work.

[anilanar]

@gcanti Is there something we can help to move this PR forward?

[gcanti]

@anilanar planned for v2.8