Added a HIDDEN flag.

[Still not a fan of the massive overlap between GenJVM and GenASM. I do
not think such duplication should be allowed into master at all, neither
in experimental backends nor anywhere else. Putting us in the position
that we have to actually switch to stop incurring the duplication is
engineering-by-prayer.]

The SYNTHETIC flag was long ago discovered to be inadequately specific
to allow for emitting ACC_SYNTHETIC in bytecode. In this commit is born
a HIDDEN flag, which signals the flagged symbol as implementation detail
which should not be considered during typechecking, and which will
receive ACC_SYNTHETIC during code generation.

Unsure what should be hidden, I conservatively marked a few things which
seem safely hidable.

 - $outer fields and accessors
 - classes whose classfile has the jvm Synthetic attribute
   (not to be confused with the SYNTHETIC flag)

I leave additional choices to those who have a better idea how
this will materialize (i.e. IDE guys.)

It is easy to selectively introduce this flag; but the SYNTHETIC flag
is set or checked for in so many places, it is very difficult to alter
the logic around it (either by setting it less, or checking for HIDDEN
only) with much confidence. So right now HIDDEN is only used to help
ACC_SYNTHETIC make it into bytecode - it is only set in conjunction with
SYNTHETIC, and it doesn't help anyone hide from the typechecker.

Review by @dragos, @odersky.
SI-6032 Enhance TaskSupport documentation.
SI-6015 assertion (in Type-flow analysis) that wasn't, valid counterexample found
Merge commit 'refs/pull/851/head' into 2.10.x
Pickled flag reorg

Cleaned up and optimized code that maps between raw and pickled flags. Avoids mystery constants. Makes a whole bunch of new flags be pickled which were not pickled before (more precisely: Everything in InitialFlags with value greater than 1 << 31 which is not in FlagsNotPickled now gets pickled whereas before it wasn't. Among these: VARARGS, IMPLCLASS, SPECIALZED, DEFAULTINIT, SYNCHRONIZED. I am curious how many tickets will get fixed by this change.

The first one I noted is t5504, which previously enforced the buggy behavior through a neg check!

There are also some build manager check file changes that have to do with the fact that flags now print in a different order for performance reasons.
starr sources to move cloneable class to package `annotation`
Moved cloneable class to package `scala.annotation`.

Requires a new starr.
SI-4763 Deprecated the `cloneable` annotation
Merge branch '2.10.x' into topic/pickledflags

Conflicts:
	src/reflect/scala/reflect/internal/Flags.scala
Removed EXISTENTIAL special case in unpickler.
Fix doc about specialized subclasses of c.i.HashMap/HashSet.

The documentation was wrong and judging from the “TODO: add HashMap2,
HashMap3, ...” in the source file I refrained from mentioning any
specific number, so overall it has gotten a bit less precise, but
hopefully more correct.

Review: @heathermiller
Merge pull request #881 from lrytz/t4763

SI-4763 deprecated `@cloneable` annotation
Merge pull request #887 from paulp/topic/pickledflags

major fixes to how flags are pickled
Merge pull request #878 from adriaanm/topic-polyvalclass

polymorphic value classes
Merge pull request #874 from adriaanm/ticket-6022

SI-6022 model type-test-implication better
Merge pull request #877 from magarciaEPFL/fixes

SI-6015 assertion (in Type-flow analysis) that wasn't, valid counterexample found
Merge pull request #884 from dcsobral/si/6032

SI-6032 Enhance TaskSupport documentation.
Optimization in asSeenFrom.

Eliminated about half the allocations of AsSeenFromMap by examining the
arguments more closely. The impact is not what one might conclude from
that statistic, because those were the cheap asSeenFrom calls - but now
they are cheaper. Also,

 - worked in some eager vals rather than lazy ones.
 - changed hot-spot comparison to use eq
 - simplified annotationArgRewriter
 - elimnated separate tracking of capturedPre

Statistics from compiling scala/collection/**.scala:

  - 4209382 calls to asSeenFrom
  - 1025945 maps created (instantiations of AsSeenFromMap)
  - 1525649 maps avoided (these were instantiations before this patch)

Review by most honorable performance-san @gkossakowski .

(Rebase of https://github.com/scala/scala/pull/835)
Further optimization of asSeenFrom.

After some more early exclusion of uninteresting arguments,
asSeenFrom is now over twice as fast.  Compiling the collections,
three runs:

  -- Without these two commits:
  8108775ms, 8132052ms, 8257527ms in asSeenFrom
  Wall clock: 0m54.202s, 0m54.877s, 0m55.925s

  -- With these two commits:
  3568719ms, 3542860ms, 3570892ms in asSeenFrom
  Wall clock: 0m50.730s, 0m51.213s, 0m50.850s

(Rebase of https://github.com/scala/scala/pull/835)
Merge pull request #885 from soc/scaladoc-hashsetmap

Fix doc about specialized subclasses of c.i.HashMap/HashSet.