[v3] Revised locale caches

[Benjamin Kosnik]

I like your comments! They are pretty funny, as good comments should be.

This looks very interesting. It looks like locale::id should just be
used in __locale_cache_base, huh? 

I like that this avoids all the gross casting caused by the ios_base vs.
basic_ios split. Big fan of this approach.

Moving to per-facet cache instead of a monolithic one would be
interesting, and would be the way to go I think. However, the
initialization issues would be more complex.

use_cache will have to be __use_cache.

I almost have timers for the performance testsuite, and when I'm done
with that I'll take a closer look at this, hopefully with memory and
performance profiling.

-benjamin