Fix Rope CharSequence .toString.

[phronmophobic]

The CharSequence returned by Rope's .toCharSequence doesn't override .toString and gives the wrong result.

> (-> (Rope/from "hello")
      .toCharSequence
      .toString)
"io.lacuna.bifurcan.Rope$1@69be5e43"

This pull request uses the same implementation that Rope's .toString implementation uses.