Adds indexing on @none, or an alias of @none for all types of map containers.

Changes behavior for id maps to use `@none` instead of a blank node identifier.

Fixes #480

Author: gkellogg

spec/latest/common/terms.html

@@ -94,9 +94,7 @@ <h1>General Terminology</h1>
       if a <a>named graph</a> is not specified.</dd>
     <dt><dfn>default language</dfn></dt><dd>
       The default language is set in the <a>context</a> using the <code>@language</code> key whose
-      value MUST be a <a>string</a> representing a [[!BCP47]] language code or <code>null</code>.
-      <span class="ednote">From <a href="#issue-480">Issue 480</a>
-      the language tag <code>@none</code> represents no language.</span></dd>
+      value MUST be a <a>string</a> representing a [[!BCP47]] language code or <code>null</code>.</dd>
     <dt><dfn>default object</dfn></dt><dd>
       A <a>default object</a> is a <a>JSON object</a> that has a <code>@default</code> member.</dd>
     <dt><dfn data-lt="edges">edge</dfn></dt><dd>