Assorted bridging changes:
• Convert _AbstractStringStorage to a protocol, and the free functions used to deduplicate implementations to extensions on that protocol.
• Move 'start' into the abstract type and use that to simplify some code
• Move the ASCII fast path for length into UTF16View.
• Add a weirder but faster way to check which (if any) of our NSString subclasses a given object is, and adopt it
