[Patch] Use std::search in string::find

[Paolo Carlini]

Dhruv Matani wrote:

>Here is a version that takes a const char*, size_type, and size_type, it
>can be adopted easily if you feel it's fine.
>  
>
Don't we agree with Matt that the first step is definitely make use of 
algorithms
in string? Afterwards, in case, we can conceive improving -slowly- the 
former...

Paolo.