Prevent annoying tooltip in Kotlin project

Module 'builtins' has no dependency on kotlin-runtime.jar, yet it has a lot of
Kotlin files. Add jet.Unit to the list of marker classes as an example of the
class not from the stdlib, but from built-ins. Since 'builtins' has jet.Unit in
its classpath, no tooltip is displayed
Extract BuiltInsSerializer to a separate module

BuiltInsSerializer is now distributed with Kotlin compiler. This will allow to
serialize binary data of built-ins on 'ant dist', as opposed to storing all
*.kotlin_class files in the repository: ant dist will just invoke this
serializer from bootstrap-compiler.jar

Also convert everything to Kotlin