\char character escaping and nicer MathML via \html@mathml (#1454)

* \html@mathml

Fix #1452

* Add missing file

* Implement \char (via internal \@char)

* Remove excess <mstyle> wrapper on \mathbin etc.

* Fix tests

* Add Unicode support for \copyright and \textregistered

Testing that this doesn't lead to an infinite loop thanks to \char` escaping.

* Add tests

* Use assertNodeType

* Switch from regex to lookup table, and no parseInt

src/functions/mclass.js

@@ -1,7 +1,7 @@
 // @flow
 import defineFunction, {ordargument} from "../defineFunction";
 import buildCommon from "../buildCommon";
-import mathMLTree from "../mathMLTree";
+import domTree from "../domTree";
 import ParseNode from "../ParseNode";
 
 import * as html from "../buildHTML";
@@ -16,7 +16,7 @@ function htmlBuilder(group, options) {
 
 function mathmlBuilder(group, options) {
     const inner = mml.buildExpression(group.value.value, options);
-    return new mathMLTree.MathNode("mstyle", inner);
+    return new domTree.documentFragment(inner);
 }
 
 // Math class commands except \mathop