stdlib: split topic-focused modules out of StdlibUnittest

The new modules are:

* SwiftUnstable -- assorted additions to the core standard library
  (more algorithms etc.)

* SwiftUnstableDarwinExtras -- proposed additions to the Darwin overlay,
  not yet reviewed by the Darwin team.

* SwiftUnstablePthreadExtras -- wrappers that make it possible to
  use pthread in Swift (they work around the lack of block-based API in
  pthread).  In future these could be possibly folded into the Darwin
  overlay as well.

These APIs are useful without StdlibUnittest for writing automation
tools in Swift.  Just like SwiftExperimental, none of these modules are
exposed to extrenal users.

Also, since these new modules can be compiled with -sil-serialize-all
(unlike StdlibUnittest, where we can't apply the flag because of
compiler bugs), standard library tests that need to run optimized code
(like AtomicInt.swift) are *much* faster now.

Swift SVN r25679

validation-test/stdlib/StringSlicesConcurrentAppend.swift

@@ -4,6 +4,7 @@
 
 import Swift
 import StdlibUnittest
+import SwiftUnstablePthreadExtras
 import Dispatch
 
 var StringTestSuite = TestSuite("String")