This adds a small string representation capable of holding up to 15
ASCII code units directly in registers. This is extendable to UTF-8 in
the future.
It is intended to be the preferred representation whenever possible
for String, and is intended to be a String fast-path. Future small
forms may be added in the future (likely off the fast-path).
Small strings are available on 64-bit, where they are most beneficial
and well accomodated by limited address spaces. They are unavailable
on 32-bit, where they are less of a win and would require much more
hackery due to full address spaces.
