Fix Scheme evaluation in fresh Emacs

* lisp/ob-scheme.el: Require geiser-impl so that
  geiser-default-implementation is defined.  Otherwise, unless Geiser
  has previously been run (e.g. with `run-geiser'), you'll see an
  error that geiser-default-implementation is undefined.

lisp/ob-scheme.el

@@ -40,6 +40,7 @@
 ;;; Code:
 (require 'ob)
 (require 'geiser nil t)
+(require 'geiser-impl nil t)
 (defvar geiser-repl--repl)             ; Defined in geiser-repl.el
 (defvar geiser-impl--implementation)   ; Defined in geiser-impl.el
 (defvar geiser-default-implementation) ; Defined in geiser-impl.el