bug#40584: R7RS string-for-each does not work two or more strings on its argument

Hi,

Actually it's my own bug but let me report here: (scheme base) lacks
implementation for string-for-each thus following code does not work;

(import (scheme base))

(string-for-each
  (lambda (e f) 'ok)
  "01234"
  "slas")

Please note that we already have incompatible string-for-each for
SRFI-13 and R6RS.
(It seems current (scheme base) uses SRFI-13 one)
R7RS for-each (and map) procedures allow different length of
collection as arguments.

Checked on 2b4e45ca1b89a942200b7b9f46060dddc44d2876