Adding moving to basic_string

[Paolo Carlini]

chris jefferson wrote:

> You are also right, this kind of stealing of internal representation
> would work on a ref-counted copy too.. so it looks like this whole
> thing is tackling a useful, but totally different problem.. woops :)

:) I agree with you and Jonathan about the first point, less about the
second ;) Really, when reference-counting doesn't help us, we have to
pay even more attention to avoiding memory allocations and memcpy.

Paolo.