fixes so that typing asdfadfasff in the repl doesn't error

Before:

> ,c (set! x 3)
   0    (make-int8 3)                   ;; 3
   2    (link "x")
   5    (variable-set)

> ,c (define x 3)
   0    (make-int8 3)                   ;; 3
   2    (link "x")
   5    (variable-set)

After:
> ,c (define x 3)
   0    (make-int8 3)                   ;; 3
   2    (define "x")
   5    (variable-set)

* src/vm_loader.c (link): `link' now errors if the variable is undefined.
  This corresponds with desired behavior, for both `ref' and `set'
  operations, for scheme. It's not what elisp wants, though. Perhaps
  elisp linking needs another instruction.
  (define): New instruction, the same as calling scm_define(), basically.

* module/language/scheme/translate.scm (trans-pair): Don't try to look up
  an existing variable definition when translating `define'; instead use
  the special-purpose lookup from ghil.scm's `ghil-define'.

* module/system/il/compile.scm (codegen): Compile to a different kind of
  variable access from `set!', specifically via passing 'define as the op
  to `make-glil-var'.

* module/system/il/ghil.scm (ghil-lookup): Don't add to the module table
  when compiling variable sets via `set!'.
  (ghil-define): New procedure, for looking up variables for `define'.

* module/system/vm/assemble.scm (<vdefine>): New record: a new
  instruction type.
  (codegen): Compile `define' module vars into <vdefine>.
  (dump-object!): <vdefine> == `define'.

module/system/vm/assemble.scm

@@ -44,6 +44,7 @@
 (define-record (<venv> parent nexts closure?))
 (define-record (<vmod> id))
 (define-record (<vlink> module name))
+(define-record (<vdefine> module name))
 (define-record (<bytespec> vars bytes meta objs closure?))
 
 
@@ -146,10 +147,16 @@
 		     (push-code! `(external-set ,(+ n index)))))))
 
 	   ((<glil-module> op module name)
-	    (push-object! (make-vlink :module #f :name name))
-	    (if (eq? op 'ref)
-		(push-code! '(variable-ref))
-		(push-code! '(variable-set))))
+            (case op
+              ((ref)
+               (push-object! (make-vlink :module module :name name))
+               (push-code! '(variable-ref)))
+              ((set)
+               (push-object! (make-vlink :module module :name name))
+               (push-code! '(variable-set)))
+              ((define)
+               (push-object! (make-vdefine :module module :name name))
+               (push-code! '(variable-set)))))
 
 	   ((<glil-label> label)
 	    (set! label-alist (assq-set! label-alist label (current-address))))
@@ -252,6 +259,9 @@
 	((<vlink> module name)
 	 ;; FIXME: dump module
 	 (push-code! `(link ,(symbol->string name))))
+	((<vdefine> module name)
+	 ;; FIXME: dump module
+	 (push-code! `(define ,(symbol->string name))))
 	((<vmod> id)
 	 (push-code! `(load-module ,id)))
         (else