Drop "Callable primary_constructor" field, because it requires serializer to
serialize the fact that a class has a primary constructor. Default primary
constructors for objects etc. may not be serialized, because they can be
re-created trivially when they're loaded. There are three options: a class
doesn't have a primary constructor, a class has a default primary constructor,
a class has a custom primary constructor. This explains the change in
descriptors.proto
This helps to avoid a nasty hack with loading inner Kotlin classes in JDR,
which makes it a bit easier to 'lazify' JDR, since now the container of a
Kotlin class is no longer required to be resolved eagerly before resolution of
the class itself
Evgeny Gerashchenko