Speed up getting non-whitespace IFS characters.

* gash/word.scm (split-fields): Compute 'char-set:ifs/nw' using
'string-delete' instead of 'char-set-difference'.

gash/word.scm

@@ -99,7 +99,7 @@ in the string @var{ifs}."
     (string->char-set ifs))
 
   (define char-set:ifs/nw
-    (char-set-difference char-set:ifs char-set:whitespace))
+    (string->char-set (string-delete char-set:whitespace ifs)))
 
   (define (wedge-apart-quote qword)
     (let loop ((qword (normalize-word qword)) (acc '()))