[ruby-core:71142] id_table cannot work for const_tbl

[Posting to hopefully prevent somebody else from going down
 the same path I did and wasting time working on a patch]

I started working on the following patch to use id_table for
const tables, but apparently ordering matters for class path name
resolution:

  http://80x24.org/spew/20151022023752.9038-1-e%4080x24.org/raw

So unless I'm missing something, I'll reject it.
All tests pass with ID_TABLE_IMPL 1 (st) with id_table, though
(but of course we want to avoid st to save memory).

WEBrick test failures:

    1) Failure:
  TestWEBrickHTTPAuth#test_basic_auth [/ruby/test/webrick/utils.rb:60]:
  exceptions on 1 threads:
  #<Thread:0x002b8597d42ab8@/ruby/test/webrick/utils.rb:46 dead>:
  /ruby/test/lib/minitest/unit.rb:200:in `assert': Expected /ERROR WEBrick::HTTPStatus::Unauthorized/ to match "[2015-10-22 02:35:17] ERROR WEBrick::HTTPAuth::Authenticator::AuthException\n". (MiniTest::Assertion)
    from /ruby/test/lib/test/unit/assertions.rb:36:in `assert'
    from /ruby/test/lib/minitest/unit.rb:292:in `assert_match'
    from /ruby/test/webrick/test_httpauth.rb:12:in `block in test_basic_auth'
    from /ruby/test/webrick/utils.rb:49:in `block in start_server'

    2) Failure:
  TestWEBrickHTTPAuth#test_basic_auth2 [/ruby/test/webrick/utils.rb:60]:
  exceptions on 1 threads:
  #<Thread:0x002b85980ebd18@/ruby/test/webrick/utils.rb:46 dead>:
  /ruby/test/lib/minitest/unit.rb:200:in `assert': webrick log doesn't have expected error: /ERROR WEBrick::HTTPStatus::Unauthorized/ (MiniTest::Assertion)
    from /ruby/test/lib/test/unit/assertions.rb:36:in `assert'
    from /ruby/test/webrick/test_httpauth.rb:41:in `block (2 levels) in test_basic_auth2'
    from /ruby/test/webrick/test_httpauth.rb:40:in `each'
    from /ruby/test/webrick/test_httpauth.rb:40:in `block in test_basic_auth2'
    from /ruby/test/webrick/utils.rb:49:in `block in start_server'

    3) Failure:
  TestWEBrickHTTPAuth#test_digest_auth [/ruby/test/webrick/utils.rb:60]:
  exceptions on 1 threads:
  #<Thread:0x002b85981beb50@/ruby/test/webrick/utils.rb:46 dead>:
  /ruby/test/lib/minitest/unit.rb:200:in `assert': webrick log doesn't have expected error: /ERROR WEBrick::HTTPStatus::Unauthorized/ (MiniTest::Assertion)
    from /ruby/test/lib/test/unit/assertions.rb:36:in `assert'
    from /ruby/test/webrick/test_httpauth.rb:119:in `block (2 levels) in test_digest_auth'
    from /ruby/test/webrick/test_httpauth.rb:118:in `each'
    from /ruby/test/webrick/test_httpauth.rb:118:in `block in test_digest_auth'
    from /ruby/test/webrick/utils.rb:49:in `block in start_server'