[Patch] Codecvt::do_out 20x faster (libstdc++/11602)

[Benjamin Kosnik]

>To tell you the truth, I'm quite proud of this work!

You should be, great job! Thanks also for adding the perfomance testcase
so that we track this issue in the future. 

>3.4 (yesterday numbers)
>-----------------------
>9.600u 0.000s 0:09.76 98.3%     0+0k 0+0io 187pf+0w
>
>3.4 + the below
>---------------
>0.590u 0.010s 0:00.60 100.0%    0+0k 0+0io 189pf+0w

Nice!

>Pretty good, eh? We are still a little bit slower than iconv
>but we can live with that, for now... Another observation:
>something similar must be prepared for codecvt::do_in, I will!

That would be appreciated.

-benjamin