Switch StringObject and StringGuts from opaquely storing tagged cocoa
strings into storing small strings. Plumb small string support
throughout the standard library's routines.
Add a visitor-like function which will inspect the bitpattern of
_StringGuts and dispatch to the appropriate mechanism. This allows us
to keep the core usage pattern in one spot, and tweak the branching
scheme as the ABI finalizes.
It also reduces the bug surface area by allowing us to maintain
resilience in the visitor, instead of by-hand at every use site. It
also prevents expression-drift, which the by-hand opaque pattern is
susceptible to.
Current implementation is very carefully written to avoid excess
ARC. Uses need to be very careful about not capturing, or else there
will be non-trivial closure contexts and performance will blow
up. Both of these aspects will hopefully be fixed soon.
