This is a giant squashing of a lot of individual changes prototyping a
switch of String in Swift 5 to be natively encoded as UTF-8. It
includes what's necessary for a functional prototype, dropping some
history, but still leaves plenty of history available for future
commits.
My apologies to anyone trying to do code archeology between this
commit and the one prior. This was the lesser of evils.
This property is too specific in that it forces a particular normalization; let's not expose it this way, but instead in the future with a full normalization API.
Restore (un-revert) sting comparison, with fixes
More exhaustive testing of opaque strings, which consistently reproduces prior sporadic failure. Shims fixups. Some test tweaking.
