0.6 2017-11-17

• Enrich the library to have all functions of the String module.
• Add rope.top findlib library so one can just do #require "rope.top";; in the toplevel (REPL).
• Compatible with -safe-string and code improvements based on string immutability.
• Port to jbuilder.
• Fix all warnings.
• Clarify the license for files in bench/ and fix FSF address.