Second beta of basic_string exponential shaper

[Paolo Carlini]

In the meanwhile I have tried http://gcc.gnu.org/ml/libstdc++/2001-07/msg00030.html also for
10000000...

mainline
--------
Execution time of 10000000 string::append(char) calls: 116.83 sec.
Execution time of 10000000 string::append(const string&) calls: 120.71 sec.

mainline + beta2
----------------
Execution time of 10000000 string::append(char) calls: 4.2 sec.
Execution time of 10000000 string::append(const string&) calls: 8.83 sec.

... not bad :-)

P.