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
Rework BuiltInsSerializer

BuiltInsSerializer will be distributed with Kotlin compiler from now on. 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