swift234

stdlib: add a private serialization API

These APIs will be used for writing automation tools in Swift.  Just
like other private APIs, this module is not exposed to extrenal users.

The primary motivation for doing instead of using NSCoder this is that
NSCoder does not work with structs and Swift containers.  Using classes
for everything just to satisfy NSCoder forces unnatural code.

This API requires two times (!) less boilerplate than NSCoding, since
the same method is used for serialization and deserialization.  This API
is also more type-safe, it does not require the user to write 'as' type
casts, unlike NSCoding.

Please take a look at
validation-test/stdlib/SwiftPrivateSerialization.swift to see the
intended use pattern.

The performance of the underlying implementation is already decent, and
there's a lot of room for improvement.

This is a re-commit of r25678, with a fix for 32-bit platforms.

Swift SVN r25877