* include/ruby/ruby.h (RUBY_INTEGER_UNIFICATION): macro to tell if
Integer is integrated. [ruby-core:75718][Bug #12427]
* include/ruby/backward.h, internal.h (rb_cFixnum, rb_cBignum):
fallback to rb_cInteger.
* bignum.c, numeric.c, ext/json/generator/generator.{c,h}: use the
macro.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55394 b2dd03c8-39d4-4d8f-98ff-823fe69b080e