Remove double encoding in MathML via Unicode MathML spaces (#1813)

* Remove double encoding in MathML via Unicode MathML spaces

We used to have a complex mechanic for escaping strings, marking them as
`needsEscape = false`, and then not escaping those strings (for
combining strings in `\operatorname`).  But this doesn't really work
with `render`ing directly to a node, as `document.createTextNode` can't
be stopped from escaping.

I've thus removed this mechanic, which required the following changes:

* Switch MathML "smart space" encoding to use Unicode instead of
  `&LongNames;` (which weren't working with `render` for the same reason).
* Hack our HTML/MathML serializer to not use `String.trim`, which wrecks
  havoc with emitted Unicode spaces.

Now `toText()` doesn't escape, so strings concatenate in unescaped form,
and `toHTML()` only does the necessary escaping.  Thus fix #1782.

* Fix src/utils.js

Co-Authored-By: edemaine <edemaine@mit.edu>

* Fix src/mathMLTree.js documentation

Co-Authored-By: edemaine <edemaine@mit.edu>

* Remove trim hack thanks to diffable-html@4.0.0

* Switch back to jest-serializer-html

* Update mathMLTree.js

test/__snapshots__/mathml-spec.js.snap

@@ -46,6 +46,7 @@ exports[`A MathML builder \\text fonts become mathvariant 1`] = `
             bold
           </mtext>
           <mtext>
+             
           </mtext>
           <mtext mathvariant="bold-italic">
             italic
@@ -68,6 +69,7 @@ exports[`A MathML builder \\text fonts become mathvariant 1`] = `
               bold
             </mtext>
             <mtext>
+               
             </mtext>
             <mtext mathvariant="sans-serif-bold-italic">
               italic
@@ -91,6 +93,7 @@ exports[`A MathML builder \\text fonts become mathvariant 1`] = `
               bold
             </mtext>
             <mtext>
+               
             </mtext>
             <mtext mathvariant="monospace">
               italic
@@ -300,7 +303,7 @@ exports[`A MathML builder should generate the right types of nodes 1`] = `
         1
       </mn>
       <mtext>
-        &ThickSpace;
+          
       </mtext>
       <mtext>
         a
@@ -344,7 +347,7 @@ exports[`A MathML builder should output \\limsup_{x \\rightarrow \\infty} correc
       <msub>
         <mo>
           <mi mathvariant="normal">
-            lim&ThinSpace;sup
+            lim sup
           </mi>
           <mo>
             ⁡
@@ -379,7 +382,7 @@ exports[`A MathML builder should output \\limsup_{x \\rightarrow \\infty} in dis
       <munder>
         <mo>
           <mi mathvariant="normal">
-            lim&ThinSpace;sup
+            lim sup
           </mi>
           <mo>
             ⁡
@@ -702,61 +705,61 @@ exports[`A MathML builder special spaces render specially 1`] = `
   <semantics>
     <mrow>
       <mtext>
-        &ThinSpace;
+         
       </mtext>
       <mtext>
-        &ThinSpace;
+         
       </mtext>
       <mtext>
-        &MediumSpace;
+         
       </mtext>
       <mtext>
-        &MediumSpace;
+         
       </mtext>
       <mtext>
-        &MediumSpace;
+         
       </mtext>
       <mtext>
-        &ThickSpace;
+          
       </mtext>
       <mtext>
-        &ThickSpace;
+          
       </mtext>
       <mtext>
-        &NegativeThinSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeThinSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeMediumSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeThickSpace;
+         ⁣
       </mtext>
       <mtext>
-        &VeryThinSpace;
+         
       </mtext>
       <mtext>
-        &ThinSpace;
+         
       </mtext>
       <mtext>
-        &MediumSpace;
+         
       </mtext>
       <mtext>
-        &ThickSpace;
+          
       </mtext>
       <mtext>
-        &NegativeVeryThinSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeThinSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeMediumSpace;
+         ⁣
       </mtext>
       <mtext>
-        &NegativeThickSpace;
+         ⁣
       </mtext>
     </mrow>
     <annotation encoding="application/x-tex">

test/setup.js

@@ -8,7 +8,7 @@ import {
     expectKaTeX, expectEquivalent,
 } from "./helpers";
 
-// Serializer support
+// JSON serializer
 
 const typeFirstCompare = (a, b) => {
     if (a.key === 'type') {