Add testing and minor refactor

This MR introduces the testing framework jest, along with some minor refactoring in the parser package, in order to be testable. I separated the basic functions into a library file, so we can test individually each function.

To try this out, follow the commands:

yarn run lerna bootstrap
yarn test

By running yarn test, lerna should automatically run all tests in all packages (right now there is 1 test on each package).

After verifying that tests are running, you should try to build the packages to see if they build correctly. (Keep in mind that the parser package builds, but it's not useful yet, it needs a little more fiddling to produce actually usable artifacts). Just run the following:

yarn build

And again, lerna should build all packages successfully.