De-genericize one axis of storage buffers to speed up compilation time
of stdlib and user code. Publish the rest as ABI, since we're out of
time to reevaluate the design.
In theory there could be a "fixed-layout" enum that's not exhaustive
but promises not to add any more cases with payloads, but we don't
need that distinction today.
(Note that @objc enums are still "fixed-layout" in the actual sense of
"having a compile-time known layout". There's just no special way to
spell that.)
Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
The best high-level APIs for decoding/transcoding are still under active
investigation. It's likely we want more views. Therefore, leave
de-underscored/public only the lowest-level APIs for now.
