/**
 * call-seq:
 *     rxp.gsub!(str, replacement)
 *     rxp.gsub!(str) {|match_data| ... }
 *     
 * Performs the substitutions of ORegexp#gsub in place, returning
 * _str_, or _nil_ if no substitutions were performed.
 *
 **/
static VALUE oregexp_m_gsub_bang(int argc, VALUE *argv, VALUE self) {