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.