Patch: improving performance of basic_string modification functions

[Benjamin Kosnik]

this is one way:

> I did a little analysis of 'basic' applications that are using strings heavily
> with 3.0.1 (release) and found that only two functions of the API are important:
[snip] 

this is another:

> Something like the stepanov abstraction penalty test would make sense, to
> at least verify that we conform to the complexity requirements of the
> standard; i.e. time adding one character to a string, then time adding a
> lot of characters one at a time and make sure that we take no more than
> linear time.

Of the two, the second seems easiest to do, off hand.

Can one or both of these be formalized in a testsuite entry?
I don't care which harness (dejagnu, mkcheck) is used.

-benjamin