* Make Range conditionally a Collection
* Convert ClosedRange to conditionally a collection
* De-gyb Range/ClosedRange, refactoring some methods.
* Remove use of Countable{Closed}Range from stdlib
* Remove Countable use from Foundation
* Fix test errors and warnings resulting from Range/CountableRange collapse
* fix prespecialize test for new mangling
* Update CoreAudio use of CountableRange
* Update SwiftSyntax use of CountableRange
* Restore ClosedRange.Index: Hashable conformance
* Move fixed typechecker slowness test for array-of-ranges from slow to fast, yay
* Apply Doug's patch to loosen test to just check for error
Changes:
- Added bounds checks to Unsafe*BufferPointer's subscript getter and setter
- Added tests for Unsafe*BufferPointer's subscript getter for all four range types
