remove all mentions of "external" from the compiler and related code

With this, GHIL is effectively bitrotten. I need to port the ECMAScript
compiler to tree-il, then I'll remove it.

* module/language/assembly.scm (byte-length):
* module/language/assembly/compile-bytecode.scm (write-bytecode):
* module/language/assembly/decompile-bytecode.scm (decode-load-program):
* module/language/assembly/disassemble.scm (disassemble-load-program):
  (disassemble-free-vars, code-annotation):
* module/language/glil.scm (<glil-program>, <glil-local>)
  (<glil-exteral>, parse-glil, unparse-glil):
* module/language/glil/compile-assembly.scm (make-meta):
  (compile-assembly, glil->assembly):
* module/language/glil/decompile-assembly.scm (decompile-toplevel):
  (decompile-load-program):
* module/language/objcode/spec.scm (decompile-value):
* module/language/tree-il/compile-glil.scm (flatten-lambda):
* module/system/vm/frame.scm (frame-binding-ref):
  (frame-binding-set!):
* module/system/vm/program.scm (binding:boxed?):
* module/system/vm/trace.scm (trace-next):
* test-suite/tests/asm-to-bytecode.test ("compiler"):
* test-suite/tests/tree-il.test: Remove all mentions of "external", and
  of <glil-local>. Docs updates will come soon.

module/language/objcode/spec.scm

@@ -66,23 +66,16 @@
    ((program? x)
     (let ((objs  (program-objects x))
           (meta  (program-meta x))
-          (exts  (program-external x))
+          (free-vars  (program-free-variables x))
           (binds (program-bindings x))
           (srcs  (program-sources x))
           (nargs (arity:nargs (program-arity x))))
-      (let ((blocs (and binds
-                        (collapse-locals
-                         (append (list-head binds nargs)
-                                 (filter (lambda (x) (not (binding:extp x)))
-                                         (list-tail binds nargs))))))
-            (bexts (and binds
-                        (filter binding:extp binds))))
+      (let ((blocs (and binds (collapse-locals binds))))
         (values (program-objcode x)
                 `((objects . ,objs)
                   (meta    . ,(and meta (meta)))
-                  (exts    . ,exts)
+                  (free-vars . ,free-vars)
                   (blocs   . ,blocs)
-                  (bexts   . ,bexts)
                   (sources . ,srcs))))))
    ((objcode? x)
     (values x #f))