Fixes for r7rs string-map, string-for-each, jiffies-per-second

R7RS string-map and string-for-each are NOT compatible to srfi-13's.
For now we implement them on top of generic map/for-each of
gauche.sequence.  Fixed jiffies-per-second (it is not a variable,
but a procedure!).  R7RS says current-second is inexact, so we
added the fraction part for subseconds.