Mark eligible functions as pure/view

[darob3rt]

Nearly all of the string utility functions can be declared as view or even pure, since they do not modify any state but only meddle with the arguments passed to them.

Please add these keywords where possible in order to enable users to keep their functions declared as view or pure without the compiler complaining that one of the called methods in the function may modify the state.

[axic]

Fixed by #25.