Improve string algorithm Stacking

[rlogel@…]

The documentation for the string algorithm library states:

"Algorithm stacking: Copy versions return a transformed input as a result, thus allow a simple chaining of transformations within one expression (i.e. one can write trim_copy(to_upper_copy(s))). Mutable versions have void return, to avoid misuse. "

I'm not sure what they mean by misuse but if all of the mutable version returned a reference to the object you are manipulating then you could chain them together without paying the price of copying the string over and over again.