Workaround being loaded alongside a different json_pure version

Fix: #646

Since both `json` and `json_pure` expose the same files, if the
versions don't match, the native extension may be loaded with Ruby
code that don't match and is incompatible.

By doing the `require json/ext/generator/state` from C we ensure
we're at least loading that.

But this is a dirty workaround for the 2.7.x branch, we should
find a better way to fully isolate the two gems.

ext/json/ext/generator/generator.c

@@ -1507,4 +1507,6 @@ void Init_generator(void)
     usascii_encindex = rb_usascii_encindex();
     utf8_encindex = rb_utf8_encindex();
     binary_encindex = rb_ascii8bit_encindex();
+
+    rb_require("json/ext/generator/state");
 }

lib/json/ext.rb

@@ -15,9 +15,6 @@ module Ext
     else
       require 'json/ext/parser'
       require 'json/ext/generator'
-      unless RUBY_ENGINE == 'jruby'
-        require 'json/ext/generator/state'
-      end
       $DEBUG and warn "Using Ext extension for JSON."
       JSON.parser = Parser
       JSON.generator = Generator

test/json/ractor_test.rb

@@ -9,7 +9,7 @@
 
 class JSONInRactorTest < Test::Unit::TestCase
   def test_generate
-    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    assert_separately(%w[-rjson -Ilib -Iext], "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
     begin;
       $VERBOSE = nil
       require "json"