Add `racket/kernel' as language with very low startup time.

collects/racket/kernel.rkt

@@ -0,0 +1,18 @@
+(module kernel '#%kernel
+  (#%require (for-syntax '#%kernel))
+
+  ;; this is duplicated from "racket/private/pre-base.rkt"
+  ;; but i'm not sure that file should require this one
+  (define-syntaxes (#%top-interaction)
+    (lambda (stx)
+      (if (eq? 'top-level (syntax-local-context))
+          'ok
+          (raise-syntax-error
+           #f
+           "not at top level"
+           stx))
+      (if (symbol? (syntax-e stx))
+          (raise-syntax-error #f "bad syntax" stx)
+          (datum->syntax stx (cdr (syntax-e stx)) stx stx))))
+
+  (#%provide (all-from '#%kernel) #%top-interaction))