Add Support for Tree shaking

[anthonyjoeseph]

Something like this PR for fp-ts - which has now been implemented in io-ts and monocle-ts as well

[m-bock]

To me it seems like the new package.json "exports" field can help a lot here.
I have not tried it yet but looks like it makes postprocessing of the dist folder obsolete. We'd only need to build a lib and es6 version. And then from one global package.json the modules can be exported like explained in the blog post.