Support Unicode Big Operators (#961)

* Support Unicode Big Operators

This is the sixth in a series of PRs to give KaTeX the ability to recognize Unicode character input.

This is the final PR in this series that will submit characters in bulk. After this, there come some characters whose alias or atom type was not obvious. I'll turn in PRs for them, too, but only after they are vetted.

* Move constants outside of main functions

* Replace ∫ with \u222b

* Add backslash

test/katex-spec.js

@@ -2634,6 +2634,10 @@ describe("Unicode", function() {
         expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿").toParse();
     });
 
+    it("should parse big operators", function() {
+        expect("∏∐∑∫∬∭∮⋀⋁⋂⋃⨀⨁⨂⨄⨆").toParse();
+    });
+
     it("should parse more relations", function() {
         expect("⊂⊃⊆⊇⊏⊐⊑⊒⊢⊣⊩⊪⊸⋈⋍⋐⋑⋔⋙⋛⋞⋟⌢⌣⩾⪆⪌⪕⪖⪯⪰⪷⪸⫅⫆").toParse();
     });