compilation enviroments are always modules; simplifications & refactorings

* module/ice-9/boot-9.scm (make-fresh-user-module): New public function,
  makes an anonymous beautified module.

* module/language/objcode/spec.scm: We used to have some things in here
  that allowed lexical variable names and values to be a part of the
  environment, but no more. Now an environment is just a module. If you
  want to "inject" free variables into code, just use lambda.

* module/language/scheme/compile-tree-il.scm (compile-tree-il): Same
  here. Also, rely on the fact that an environment *will* be a module --
  because (system base compile) guarantees that for us.

* module/language/scheme/spec.scm (scheme): In the reader, rely on the
  environment being a module. Define a #:make-default-environment
  handler, which returns a beautified module, augmented with a fresh
  definition for current-reader, so that side effects to current-reader
  are restricted to the compilation unit.

* module/language/tree-il/analyze.scm
  (report-possibly-unbound-variables):
* module/language/tree-il/compile-glil.scm (compile-glil):
* module/language/tree-il/optimize.scm (optimize!): The environment will
  be a module.

* module/system/base/language.scm (<language>): New field,
  `make-default-environment'. Defaults to `make-fresh-user-module'.
  (default-environment): New accessor, returns a default environment for
  a language.

* module/system/repl/common.scm (repl-compile): Always compile relative
  to the current module, because a module is always acceptable as an
  environment.

* module/system/base/compile.scm (compile-file, compile-and-load): Both
  of these have a new keyword argument, #:env. For `compile-file', it
  defaults to the default environment of the source language, and for
  `compile-and-load', to the current module.
  (read-and-compile): If there are no expressions read, pass the joiner
  its default environment (via `default-environment joint').

module/language/objcode/spec.scm

@@ -22,25 +22,17 @@
   #:use-module (system base language)
   #:use-module (system vm objcode)
   #:use-module (system vm program)
-  #:export (objcode make-objcode-env))
-
-(define (make-objcode-env module externals)
-  (cons module externals))
-
-(define (objcode-env-module env)
-  (if env (car env) (current-module)))
-
-(define (objcode-env-externals env)
-  (and env (vector? (cdr env)) (cdr env)))
+  #:export (objcode))
 
 (define (objcode->value x e opts)
-  (let ((thunk (make-program x #f (objcode-env-externals e))))
-    (if e
+  (let ((thunk (make-program x #f #f)))
+    (if (eq? e (current-module))
+        ;; save a cons in this case
+        (values (thunk) e e)
         (save-module-excursion
          (lambda ()
-           (set-current-module (objcode-env-module e))
-           (values (thunk) #f e)))
-        (values (thunk) #f e))))
+           (set-current-module e)
+           (values (thunk) e e))))))
 
 ;; since locals are allocated on the stack and can have limited scope,
 ;; in many cases we use one local for more than one lexical variable. so