use NUM2SIZET and SIZET2NUM

* configure.in: revert r41106.  size_t may not be unsigned

* bignum.c (rb_absint_size_in_word, rb_int_export, rb_int_import): use
  NUM2SIZET() and SIZET2NUM() already defined in ruby/ruby.h.

* ext/-test-/bignum/export.c (rb_int_export_m): ditto.

* ext/-test-/bignum/import.c (rb_int_import_m): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41130 b2dd03c8-39d4-4d8f-98ff-823fe69b080e