Add a use an unchecked subscript on UnsafeBufferPointer, which skips
debugPrecondition checks (in case we're not inlined) as well as a
force-unwrap check.
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 and rename _StringGutsSlice, apply NFC-aware fast paths to a
new buffered iterator.
Also, fix bug in _typeName which used to assume ASCIIness and better
SIL optimizations on StringObject.
* 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.
