The module rcstring provides everything necessary to replace D's build-in strings with a reference counted version. It should be as easy as replacing all occurrences of the keyword string with String.
Additional, to String there are WString and DString defined in this module.
String str = "Hello World";
assert(str.front == "H");
assert(str.back == "d");
auto slice = str[1 .. 5];
assert(slice == "ello");
slice.popFront();
assert(slice == "llo");