[Patch] (Almost) fix libstdc++/11723

[Paolo Carlini]

Hi,

this (almost) fixes the PR, in that we don't really achieve the
same speed of C, but a very good improvement with a minor cost
in terms of memory consumption and overhead at construction time.
I think it's ok for now. The approach is the same already used
for <wchar_t>::do_narrow/do_widen.
These are the numbers:

3.4
---
15.960u 0.020s 0:16.56 96.4%    0+0k 0+0io 175pf+0w

3.4 + patch
-----------
 1.630u 0.000s 0:01.69 96.4%    0+0k 0+0io 175pf+0w

Tested x86-linux (gnu/generic). Will wait 'til tomorrow (italian
time) for comments...

Paolo.

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