Speeding up v3 basic_string

[Benjamin Kosnik]

> I think we need to find some way of putting these performance-based
> testcases in the repository.  :-)

Seconded. Putting all this stuff in is ok, but putting it in and 
augmenting the testing infrastructure to make sure that the performance 
gains remain is a much more clever hack.

This is an outstanding issue.

> We don't want them to run as part of 'make check' since they could take
> several minutes all together, right?  Maybe 'make perf' or some such.

We might as well make them part of 'make check.'

I was thinking that a directory, say 'benchmark' or 'performance' or 
'performance_analysis' could be added to the testsuite directory.

-benjamin