Generate ES imports/exports to enable tree shaking

[davidyuk]

purescript planning to introduce it in the next major release: purescript/purescript#3613

Until that we can try to rewrite require/module.exports by ourselves this way: https://github.com/jameswomack/replace-require-with-import/blob/master/index.js

Also, would be nice to import not the whole file, but only used parts, like

import { falsifyErrors as Erlang_Helpers_falsifyErrors } from "../Erlang.Helpers";

instead of

import Erlang_Helpers from "../Erlang.Helpers";

Tree shaking should significantly reduce the amount of imported code if the user needs one specific function, not the whole compiler.

[gorbak25]

We started out with a tree shaking in mind but we unfortunately get rid of it at one point :(
The main issue is with circular imports - they are commonplace in erlang but strictly forbidden in purescript :(
In Erlang you can have a function in module A calling a function in module B calling a function in module A.
To work around it remote calls are being dispatched dynamically via a code server - https://github.com/erlscripten/erlps-core/blob/2be641733d1179a6198835f917c3d3cbbc820712/es6/ffi_es6_builtins.js#L998
This unfortunately is not optimal and incurs a performance penalty - the upside is that this allows us to match erlang's semantics closely and weird OTP stuff like on_load callbacks or hot code reloading are feasible to implement.

@radrow ES6 modules allow for circular imports - let's investigate whether we can make tree shaking working :)