recompiling with compile environments, fluid languages, cleanups

* ice-9/boot-9.scm (compile-time-environment): Remove definition from
  boot-9 -- instead, autoload it and `compile' from (system base
  compile).

* libguile/objcodes.h:
* libguile/objcodes.c (scm_objcode_to_program): Add an optional argument,
  `external', the external list to set on the returned program.

* libguile/vm-i-system.c (externals): New instruction, returns the
  external list. Only used by (compile-time-environment).

* libguile/vm.c (scm_load_compiled_with_vm): Adapt to
  scm_objcode_to_program change.

* module/language/scheme/translate.scm (translate): Actually pay
  attention to the environment passed as an argument.
  (custom-transformer-table): Expand out (compile-time-environment) to
  something that can be passed to `compile'.

* module/system/base/compile.scm (*current-language*): Instead of
  hard-coding `scheme' in various places, use a current language fluid,
  initialized to `scheme'.
  (compile-file, load-source-file): Adapt to *current-language*.
  (load-source-file): Ada
  (scheme-eval): Removed, no one used this.
  (compiled-file-name): Don't hard-code "scm" and "go"; instead use the
  %load-extensions and %load-compiled-extensions.
  (cenv-module, cenv-ghil-env, cenv-externals): Some accessors for
  compile-time environments.
  (compile-time-environment): Here we define (compile-time-environment)
  to something that will return #f; the compiler however produces
  different code as noted above.
  (compile): New function, compiles an expression into a thunk, then runs
  the thunk to get the value. Useful for procedures. The optional second
  argument can be either a module or a compile-time-environment; in the
  latter case, we can recompile even with lexical bindings.
  (compile-in): If the env specifies a module, set that module for the
  duration of the compilation.

* module/system/base/syntax.scm (%compute-initargs): Fix a bug where the
  default value for a field would always replace a user-supplied value.
  Whoops.

* module/system/il/ghil.scm (ghil-env-dereify): New function, takes the
  result of ghil-env-reify and turns it back into a GHIL environment.

* scripts/compile (compile): Remove some of the tricky error handling, as
  the library procedures handle this for us.

* test-suite/tests/compiler.test: Add a test for the dynamic compilation
  bits.

module/system/base/syntax.scm

@@ -87,7 +87,10 @@
       (error "too many initargs" args slots))
      (else
       (lp (cdr in) (cdr positional)
-          (acons (car positional) (car in) out))))))
+          (let ((slot (car positional)))
+            (acons (if (pair? slot) (car slot) slot)
+                   (car in)
+                   out)))))))
 
 (define-macro (record-case record . clauses)
   (let ((r (gensym)))