Named references, a trilogy in five parts.

Based on discussion with Daniel, here are code and test updates to
applied following the first three named reference patches I sent on
Feb 5th.  Some notes:

The zsh/parameter module has been updated to be able to "see" named
references.  That is, whereas
  typeset -n nameref=var
  print ${(t)nameref}
outputs the same string as ${(t)var}, using
  print ${parameters[nameref]}
outputs the equivalent of
  nameref-${(t)var}

The "bad substitution" error on
  typeset -n nr='ary[2]'
  print ${nr}
has been fixed, and a problem with find the correct scope for a local
nameref also corrected.

Tests for these have been added, based on the examples Daniel provided
and a couple more.  I also changed the autoload test to be skipped if
the module isn't loadable.