add repl option to interpret rather than compile

* module/language/scheme/spec.scm (scheme): Specify an evaluator, `eval'.

* module/system/repl/common.scm (repl-default-options): Add option,
  `interp', specifying that, if possible, the repl should interpret its
  expressions rather than compile them. Defaults to #f.

module/language/scheme/spec.scm

@@ -46,5 +46,6 @@
   #:reader	read
   #:read-file	read-file
   #:translator	translate
+  #:evaluator	eval
   #:printer	write
   )

module/system/repl/common.scm

@@ -38,7 +38,8 @@
 (define-record (<repl> vm language options tm-stats gc-stats vm-stats))
 
 (define repl-default-options
-  '((trace . #f)))
+  '((trace . #f)
+    (interp . #f)))
 
 (define %make-repl make-repl)
 (define (make-repl lang)
@@ -68,7 +69,9 @@
 
 (define (repl-eval repl form)
   (let ((eval (language-evaluator (repl-language repl))))
-    (if eval
+    (if (and eval
+             (or (not (language-translator (repl-language repl)))
+                 (assq-ref (repl-options repl) 'interp)))
 	(eval form (current-module))
 	(vm-load (repl-vm repl) (repl-compile repl form)))))