Modulized API

[Cylix]

From #304 (iota.lib.js changes):

Modularized from api, utils, crypto, multisig & valid to packages seen here: https://www.npmjs.com/org/iota - Everything is in iota.js monorepo, tested in integration and released with an even version across packages. Monorepos are really cool in our case! Please review, it's important to have high cohesion & low coupling! (would be nice to look for some good static code analysis tools too...)

Their code structure is pretty interesting with a repo split into independent modules.
We can probably do the same on our side in the future. We would then need to understand what each module is made for and what modules we can provide as well. We would also need to understand how to re-arrange our buildchain.

[thibault-martinez]

I strongly advise the use of https://bazel.build/ for this one