Add mima exclude filters for mutable.Stack
Add mima exclude filters for mutable.Stack

Scala 2.12.2 produced a lot more warnings about deprecation.  One of
them that affected scala-xml suggested dropping mutable.Stack in favor
of immutable.List and var.

I tried to preserve binary compatibility by creating members that use
the collection.mutable.Stack.apply factory method, since it won't
produce deprecation warnings. Must be only the constructor has
deprecation warnings? Regardless, I was committing fraudulent binary
compatibility: I created members of the type mima wants, but the
values are not actually used.

In all likelihood, no one uses the members of FactoryAdapter.  We
will just change everything to List, drop the use of mutable.Stack
entirely, break bincompat, add entries to mimaBinaryIssueFilters,
inform users, and call it fixed.

ashawley/scala-xml