Ruby

• bignum.c (big2str_karatsuba): Use bigdivrem_restoring directly to
  reduce working buffer and memory copy.
  (rb_big2str1): Allocate working buffer for big2str_karatsuba here.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42566 b2dd03c8-39d4-4d8f-98ff-823fe69b080e