Use Melange

[jchavarri]

Updates the project to use Melange.

• tomorrow becomes a "universal" library, shared by native and Melange builds
• yesterday keeps being interface shared / impl specific, by copying the mli into the specific platform folders
• date libs is just rearranged to work on both platforms

Besides that:

• all the npm / js specific stuff is removed
• esy is replaced with opam

Edited, old description: This PR is an exploration to see if it's possible to use simple copy# rules to share code between Melange and native libraries.

The idea is to put all shared code in a separate folder, and then add rules in the platform specific library like:

(rule
 (target tomorrow.ml)
 (action
  (copy# ./../shared/tomorrow.ml tomorrow.ml)))

One can also copy interfaces in case both implementation libs need to comply with some mli.

The main downsides are:
- boilerplate to create the copying rules
- even if merlin can provide some type information in the shared code if it is built, it is expected to be failing as the cm*s of the modules in the implementation won't be available in the shared folder