This takes care of the standard library portion, but we need a new
BuiltinUTF16ExtendedGraphemeClusterLiteralConvertible protocol in order to
fully recover the performance of character literals.
Note that part of the character_literals.swift test is currently disabled. That
will need to be fixed before we can merge this work.
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.
This is a follow-up fix for making struct constructors inline(__always) in
155db0a4bd: Let Character literals, which fit into 64 bits, be folded into a single integer constant.
and
d8f1caf4a6: Inline all the new low-level bits
If we decide that this structs should not have fixed layout we must re-evaluate the performance difference of not being able to inline
the struct constructors.
