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/language/scheme/translate.scm

@@ -191,7 +191,8 @@
      (pmatch tail
        ;; (define NAME VAL)
        ((,name ,val) (guard (symbol? name) (ghil-env-toplevel? e))
-        (make-ghil-define e l (ghil-lookup e name) (trans:x val)))
+        (make-ghil-define e l (ghil-define (ghil-env-parent e) name)
+                          (trans:x val)))
 
        ;; (define (NAME FORMALS...) BODY...)
        (((,name . ,formals) . ,body) (guard (symbol? name))