ITERATE version 1.6.0 released

This is primarily a bug fix release, but it introduces `with-hash-table` as alternative (and more in line with CL) syntax for `with-hashtable`, so I bumped the minor release, since anything that uses this new syntax will not work with 1.5.x or earlier. Thanks to our contributors, and hope you enjoy the new ITERATE. I will see about getting Quicklisp updated.