[ruby/json] Refactor JSON::Ext::Parser to split configuration and parsing state

Ref: https://github.com/ruby/json/pull/718

The existing `Parser` interface is pretty bad, as it forces to
instantiate a new instance for each document.

Instead it's preferable to only take the config and do all the
initialization needed, and then keep the parsing state on the
stack on in ephemeral memory.

This refactor makes the `JSON::Coder` pull request much easier to
implement in a performant way.

c8d5236a92

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

test/json/json_ext_parser_test.rb

@@ -6,11 +6,11 @@ class JSONExtParserTest < Test::Unit::TestCase
 
   def test_allocate
     parser = JSON::Ext::Parser.new("{}")
-    assert_raise(TypeError, '[ruby-core:35079]') do
-      parser.__send__(:initialize, "{}")
-    end
+    parser.__send__(:initialize, "{}")
+    assert_equal "{}", parser.source
+
     parser = JSON::Ext::Parser.allocate
-    assert_raise(TypeError, '[ruby-core:35079]') { parser.source }
+    assert_nil parser.source
   end
 
   def test_error_messages