The attached patch does print and read of hashtables.  It respects
print-length only, exactly like vector print and consistent with the
rest of GNU Emacs.  Please review and comment.  Some notes:

I ended up with this code to put a list into Fmake_hash_table format
('params' is a Lisp_Object that holds a bunch of conses).

	      int param_count = 0;
	      Lisp_Object *pa = (Lisp_Object*) xmalloc(XFASTINT(Flength(params)) * sizeof(Lisp_Object));

	      while (!NILP(params))
		{
		  pa[param_count++] = CAR_SAFE(params);
		  params = CDR_SAFE(params);
		}

	      Lisp_Object ht = Fmake_hash_table(param_count, pa);

I would prefer it if I could avoid all that work, but it seems (from
reading lisp.h and hunting for similar code) that it must be done as
above.  I don't see a macro to convert a Lisp_Object that holds a list
into the Lisp_Object* that a lot of functions need.

Also I don't know if what xmalloc() gives needs to be freed eventually.

To generate the Lisp symbol ":size" from the symbol "size" for example,
I used:

Lisp_Object colon = make_string (":", 1);
intern(SDATA(concat2(colon, SYMBOL_NAME(head))))

I don't know if there's a better way, but this works.

Thanks!
Ted