Implement method `CppStringT::format()`

[schmouk]

" str.format(*args, **kwargs)

Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces {}. Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument."

Notice: may be implemented later.

[schmouk]

Completed. Not that immediate. Still to be tested.