- Add a $flags parameter to ZipArchive::openString(), by analogy with
  ZipArchive::open(). This allows the string to be opened read/write.
- Have the $data parameter to ZipArchive::openString() default to an
  empty string, for convenience of callers that want to create an empty
  archive. This works on all versions of libzip since the change in
  1.6.0 only applied to files, it's opt-in for generic sources.
- Add ZipArchive::closeString() which closes the archive and returns
  the resulting string. For consistency with openString(), return an
  empty string if the archive is empty.
- Wrap strings passed to libzip with zip_source_function_create()
  instead of using zip_source_buffer_create(). This allows us to make
  the string writable, and simplifies memory management.