After rebasing on master and incorporating more 32-bit support,
perform a bunch of cleanup, documentation updates, comments, move code
back to String declaration, etc.
* Refactor out RRC implementation into dedicated file.
* Change our `_invariantCheck` pattern to generate efficient code in
asserts builds and make the optimizer job's easier.
* Drop a few Bidi shims we no longer need.
* Restore View decls to String, workaround no longer needed
* Cleaner unicode helper facilities
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.
On Windows, `__swift_stdlib_UNumericType` and `__swift_stdlib_UCharCategory` are imported as `Int32`s rather than `UInt32`. Change the constructors to use the type's inferred `RawValue` rather than always `UInt32`.
- numericValue returns nil instead of .nan for non-numerics
- Remove small-string optimizations from _scalarName that failed on 32-bit archs
- Put case mappings back into U.S.Properties
- Added more sanity tests
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.
