modified:   iterators.lsp

issue #1
Following enhancements have been added to the current version
1. Extending the compatibility with TCR.SEQUENCE-ITERATORS,
   a. renaming some class members, viz the api macros and the iterator
      class members are in sync(end -> length, values -> initial-contents).
   b. adding another class-member, initial-element, which will be used over
      index, incase initial-contents are not provied
   c. Adding implementation for the following macros from the API
      i.   do-sequence-iterator
      ii.  do-sequence-iterators
      iii. do-sequence-iterators*
      iv.  dosequences*
      v.   dosequence

2. Removing to-string from the exported set of functions from the package.
Merge branch 'master' of https://github.com/ykm/cl-iterator