Add ,width meta-command to set screen width in debug output

Add ,width meta-command to set screen width in debug output

This meta-command allows one to set the default number of columns
that output from ,backtrace and ,locals shall occupy.

* doc/ref/scheme-using.texi (Debug Commands): document ,width
* module/system/repl/command.scm (*width*): new var
  (backtrace, locals): use *width* in optarg
  (width): new meta-command