Add support for ScalaJS

[zainab-ali]

This follows from a discussion with @cquiroz about Libra adoption.
Potentially many users need ScalaJS support.

Spire, shapeless and singleton-ops all support this, so it should be trivial to add.

[zainab-ali]

ScalaJS would make the build matrix much larger, and consequently would cause an explosion in build times. I'd be happy to support this, but only if someone actually has a use for it.

If anyone does want to use libra with ScalaJS, please comment here.

[cquiroz]

Build times tend to be larger for scala.js in any case and i guess it will double the matrix size right? i haven't followed this up as i wasn't able to update singleton-ops but now that you did I may give it a try

[zainab-ali]

Yes, it would double the build times. Our build times are pretty small compared to most projects (5 - 6 minutes), so I'm happy to double them as long as someone has a use case. Adding ScalaJS should be fairly straightforward.

[cquiroz]

The changes aren't that big but i'm having troubles running tests on the JS side. I'll let you know if I can make it work

[maekl]

I needed this and have a working implementation apart from a single failing test. Double.toString does not behave the same way in ScalaJS as on the jvm. It can be solved using platform-specific code (see for example how it was solved in squants)

The new directory layout would be:

.
├── core
        ├── js
        ├── jvm
        └── shared

If this sounds ok I'd be happy to wrap it up in a PR

---

I have submitted an idea (#108) which I think would provide another way to solve the platform differences (with the help of scala-java-locales)

[maekl]

Turns out platform specific code isn't required. Doubles just have to be formatted with something like "%.1f".format(d). Not easily done with the current code without something like #108 AFAICT

An alternative view is that nice & predictable formatting isn't (at least not yet) an important feature. After all, (0.1d + 0.2d).toString == "0.30000000000000004" is true. So, it might make sense to just update the failing test to use a number that produces the same result on both platforms.

[zainab-ali]

Thanks @maekl ! As stated in #108 , I think we could do with a ShowMagnitude typeclass.

I'd be very happy to include ScalaJS. Please feel free to submit a PR for it.

[maekl]

I have submitted a PR. However, I did run into a few issues, mostly around the microsite in combination with cross-compilation. Tried fixing them with minimal dependency upgrades but ran into even more incompatibility issues. I could only make it work by upgrading the Scala versions as well as many other dependencies of the project.

[zainab-ali]

This has been merged in 🎉
I'm aiming for a release later today

[zainab-ali]

This has been release in v0.7.0. Thanks @maekl !

Please feel free to open a PR and add yourself to the list of contributors