Change to DEFSTRUCT

[MOON at MIT-AI (David A. Moon)]

In Lisp machine system 16 the named-structure feature will be changed.
The :NAMED-STRUCTURE option to DEFSTRUCT has been flushed, and renamed
to :NAMED.  (The rename is to ensure that code which has not been
converted to the new syntax will be detected and signal an error.)  The
slot in the structure for the "named structure symbol" is now
automatically reserved and initialized (formerly it was documented to
be but it wasn't).

The documentation in the manual on named structures is completely poor.
I haven't revised it yet but intend to do so soon.

An additional point to note is that if you have calls to defstruct that
don't have colons in front of the keywords, you should put some in,
since at some point they will be required.