Refactorings towards a potential compiler plugin.
Rely on .class files instead of .jstype files for type info.

The tools in the Scala ecosystem are terribly dependent on
finding .class files as output, and nothing else. So after
months of trying to avoid spending useless work on
generating .class files (and instead emitting only pickles
in .jstype files), I give up and simply let the Scala
compiler generate .class files.

Scala.js is thus merely an additional phase between mixin
and cleanup, which outputs .js files only. The compiler
then proceeds as usual and emits .class files.
Make Scala.js a compiler plugin!

Yeah! Having abandoned all pretense with .jstype files,
it is now easy to turn Scala.js into a simple compiler
plugin, since it's simply an additional phase in the
pipeline.