Fixed a typo in docstring for progress in src/conditions.lisp

* src/conditions.lisp (progress): fixed typo in documentation string
Added progressing function in src/conditions.lisp

* src/conditions.lisp (progressing): new function; helper function for
  signaling progress condition for higher-order sequence functions
* src/package.lisp (package more-conditions): added exported symbol
  progressing
* test/macros.lisp (test with-sequence-progress-root::smoke): added
  cases for `progressing'
* more-conditions.asd (+version-revision+): bumped version 1 -> 2
* README.org (Tracking and Reporting Progress of Operations): mention
  `progressing'
Warn about performance impact of progress conditions in README.org

* README.org (Tracking and Reporting Progress of Operations): warn
  about performance impact