SI-7296 Remove arity limit for case classes

When venturing above the pre-ordained limit of twenty
two, `Companion extends FunctionN` and `Companion.unapply`
are sacrificed. But oh-so-many other case class features
work perfectly: equality/hashing/stringification, the apply
method, and even pattern matching (which already bypasses
unapply.)

There was some understandable fear of the piecemeal when
I tabled this idea on scala-internals [1]. But I'd like
to persist as this limit is a needless source of pain for
anyone using case classes to bind to database, XML or JSON
schemata.

[1] https://groups.google.com/forum/#!topic/scala-internals/RRu5bppi16Y