Deprecates Par*View and Gen*View

In 2.12, this gives us the option to move the code from
Gen*View down into *View. If we don't do something more
drastic with views, which inertia and history suggests
is a real possibility, we can at least shed a little of
the implementation.

These abstractions are *only* used to share implementation;
there is no `view` method available on, for instance, `GenSeq`
that lets one abstract over parallel/sequential collections
while spawning views.

  scala> (List(1): collection.GenSeq[Int]).view
  <console>:8: error: value view is not a member of scala.collection.GenSeq[Int]
                (List(1): collection.GenSeq[Int]).view
                                                  ^
Let's keep it that way.

I suspect that views over parallel collections exist not because
they were the most sought after feature, but rather because the
initial incarnatin of parallel collections used to live undernead
TraversableOnce, and hence were obligated to implement `def view`.

This change will give us deprecation warnings in the non deprecated
places that extend `Gen*View` (three, by my count) in the interim.
There are ways to avoid this, but they aren't particularly appealing.
Removes Gen*View and Par*View

 - code that used to be inherited in *View is now inlined
 - the `view` methods on `ParIteratoa` and `ParSeq` now
   convert to sequential collections, and are deprecated
   asking the user to do this explicitly in the future.

Should be largely source compatible with 2.10.x, on the assumption
that the removed classes, while being public, were internal
implementation details.

A few tests used now-removed classes to demonstrate compiler crashes.
I managed to confirm that after my decoupling, t4365 still exercises
the bug:

	% qbin/scalac test/files/pos/t4365/*.scala
	warning: there were 2 deprecation warning(s); re-run with -deprecation for details
	one warning found
	% scalac-hash 7b4e450 test/files/pos/t4365/*.scala
	warning: there were 2 deprecation warning(s); re-run with -deprecation for details
	one warning found
	% scalac-hash 7b4e450~1 test/files/pos/t4365/*.scala 2<&1 | grep -i wrong
	error: something is wrong: cannot make sense of type application
	something is wrong: cannot make sense of type application
	something is wrong: cannot make sense of type application

I didn't manage to do the same for specializes-sym-crash.scala,
and instead just made it compile.
Merge pull request #3191 from retronym/ticket/deprecate-par-view

Remove parallel collection views and, with them, Gen*View