Implement \neq and \notin via macros like LaTeX's (#1400)

* Implement \neq and \notin via macros like LaTeX's

* Fix test

test/katex-spec.js

@@ -1404,13 +1404,12 @@ describe("A font parser", function() {
         expect(nestedParse.value.font).toEqual("mathbb");
         expect(nestedParse.value.type).toEqual("font");
 
-        expect(nestedParse.value.body.value.length).toEqual(3);
+        expect(nestedParse.value.body.value.length).toEqual(4);
         const bbBody = nestedParse.value.body.value;
         expect(bbBody[0].type).toEqual("mathord");
-        expect(bbBody[1].type).toEqual("rel");
-        expect(bbBody[2].type).toEqual("font");
-        expect(bbBody[2].value.font).toEqual("mathrm");
-        expect(bbBody[2].value.type).toEqual("font");
+        expect(bbBody[3].type).toEqual("font");
+        expect(bbBody[3].value.font).toEqual("mathrm");
+        expect(bbBody[3].value.type).toEqual("font");
     });
 
     it("should work with \\textcolor", function() {

test/screenshotter/ss_data.yaml

@@ -224,7 +224,8 @@ NewLine: |
 Not: |
     \begin{array}{l}
         \not=\not>\not\geq\not\in\not<\not\leq\not{abc} \\
-        \not xy + ab \not xy
+        \not xy + ab \not xy \\
+        a \neq b \notin c
     \end{array}
 NullDelimiterInteraction: a \bigl. + 2 \quad \left. + a \right)
 OldFont: |