This removes the `_core` property from UnicodeScalarView.Index
and moves any remaining index-moving logic from the index to
the view in UnicodeScalarView and CharacterView.
This documentation revision covers a large number of types & protocols:
String, its views and their indices, the Unicode codec types and protocol,
as well as Character, UnicodeScalar, and StaticString, among others.
This also includes a few small changes across the standard library for
consistency.
The RangeProtocol was a very weak and fragile abstraction because it
didn't specify the interpretation of the endpoints. To write a
non-trivial algorithm, one usually needed to consult that information.
The standard library code only actually worked correctly with half-open
and closed ranges (and didn't handle fully open ranges, for example).
The other two protocols, HalfOpenRangeProtocol and ClosedRangeProtocol,
were only used for code sharing, and present an ABI burden. We can use
gyb instead.
