Tag everything in the standard library with accessibility attributes.

Keep calm: remember that the standard library has many more public exports
than the average target, and that this contains ALL of them at once.
I also deliberately tried to tag nearly every top-level decl, even if that
was just to explicitly mark things @internal, to make sure I didn't miss
something.

This does export more than we might want to, mostly for protocol conformance
reasons, along with our simple-but-limiting typealias rule. I tried to also
mark things private where possible, but it's really going to be up to the
standard library owners to get this right. This is also only validated
against top-level access control; I haven't fully tested against member-level
access control yet, and none of our semantic restrictions are in place.

Along the way I also noticed bits of stdlib cruft; to keep this patch
understandable, I didn't change any of them.

Swift SVN r19145

stdlib/core/StringCore.swift

@@ -24,7 +24,7 @@
 // size with the ternary operator.  This is also the cause of the
 // extra element requirement for 8 bit elements.  See the
 // implementation of subscript(Int) -> UTF16.CodeUnit below for details.
-struct _StringCore {
+@public struct _StringCore {
   //===--------------------------------------------------------------------===//
   // Internals
   var _baseAddress: COpaquePointer