[WIP] Cross-compile core, testkit and boopickle macro to scala.js

[vpavkin]

PoC of scala.js build. (originated in #54 )

@SemanticBeeng Here you go.

Just have to setup runtime for js tests and maybe separate platform independent tests into a cross-project.

You can pick up from here if you want.

[SemanticBeeng]

@vpavkin - thank you very much! :-)

Fyi @karolchmist.