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/system/base/language.scm

@@ -25,10 +25,10 @@
             language-name language-title language-version language-reader
             language-printer language-parser 
             language-compilers language-decompilers language-evaluator
-            language-joiner
+            language-joiner language-make-default-environment
 
             lookup-compilation-order lookup-decompilation-order
-            invalidate-compilation-cache!))
+            invalidate-compilation-cache! default-environment))
 
 
 ;;;
@@ -45,7 +45,8 @@
   (compilers '())
   (decompilers '())
   (evaluator #f)
-  (joiner #f))
+  (joiner #f)
+  (make-default-environment make-fresh-user-module))
 
 (define-macro (define-language name . spec)
   `(begin
@@ -97,3 +98,8 @@
                       reverse!)))
           (set! *decompilation-cache* (acons key order *decompilation-cache*))
           order))))
+
+(define (default-environment lang)
+  "Return the default compilation environment for source language LANG."
+  ((language-make-default-environment
+    (if (language? lang) lang (lookup-language lang)))))