Emit serialized IR files instead of .js + .js.map + .sjsinfo.

The ClasspathBuilder automatically expands .sjsir files into their
desugared VirtualScalaJSClassfile version for now. It is left for
a future commit to get rid of VirtualScalaJSClassfile entirely and
have the packager and optimizer work directly on the IR file.
Remove VirtualScalaJSClassfile and use VirtualScalaJSIRFile instead.

Now the various tasks working with a ScalaJSClasspath extract the
information they need from the IR directly, and work with that.
Cache the desugared pieces of classes in the fast optimizer.

This way, info and trees of .sjsir files that have not changed
since the last invocation of fastOptJS need not be reloaded from
disk. Instead, they are kept in-memory. For the trees, the
already desugared is stored (and only pieces actually used).