flush whitespace from the repl input buffer *before* evaluation

flush whitespace from the repl input buffer *before* evaluation

* module/system/repl/repl.scm (start-repl): Given that the input port of
  the repl is line-buffered, it's likely we have #\newline in the input
  that is strictly extraneous, an in-band indicator to the repl that it
  should begin reading now. So flush out that newline, so that you can
  (read-char) at the repl, and it actually does wait for you to type in
  a char instead of just returning #\newline.

  While it's not an overriding concern, this does fix some brainfuck
  programs that want to input from the user.