Make /lib in deep import paths optional

[slikts]

Enables importing modules as import { Either } from 'fp-ts/Either' instead of 'fp-ts/lib/Either' while staying backward compatible. It publishes from /lib instead of the package root, copies and patches package.json to /lib, and copies all generated source files to /lib/lib to keep backward compatibility.

The changes are intended to be minimal, but one caveat is that npm-link also needs to run from the /lib directory to work, and that changes in the copied files (/lib/lib and /lib/package.json) won't be updated by using tsc watch mode.

[gcanti]

Closing in favor of #1053