Fix space handling (#912)

Fixes several issues with space handling: (fix #910)
1. "Control symbols" (as they're called in the TeXbook), such as `\\`, should
   not have spaces eaten after them (only "control words" such as `\foo`).
2. In math mode, spaces should be consumed at the parser level, not the
   gullet level.  This enables `\\ [x]` to parse differently from `\\[x]`
3. Eat spaces between arguments, so `\frac x y` still works.
   (This used to work only because math mode ate all spaces.
    The analog in text mode wouldn't have worked.)

Also eat spaces in initial arguments in math mode, and before ^ and _ in atoms.

test/katex-spec.js

@@ -210,9 +210,11 @@ describe("A parser", function() {
     });
 
     it("should ignore whitespace", function() {
-        const parseA = stripPositions(getParsed("    x    y    "));
-        const parseB = stripPositions(getParsed("xy"));
-        expect(parseA).toEqual(parseB);
+        expect("    x    y    ").toParseLike("xy");
+    });
+
+    it("should ignore whitespace in atom", function() {
+        expect("    x   ^ y    ").toParseLike("x^y");
     });
 });
 
@@ -2397,6 +2399,16 @@ describe("An aligned environment", function() {
             .toParse();
     });
 
+    it("should allow cells in brackets", function() {
+        expect("\\begin{aligned}[a]&[b]\\\\ [c]&[d]\\end{aligned}")
+            .toParse();
+    });
+
+    it("should forbid cells in brackets without space", function() {
+        expect("\\begin{aligned}[a]&[b]\\\\[c]&[d]\\end{aligned}")
+            .toNotParse();
+    });
+
 });
 
 describe("A parser that does not throw on unsupported commands", function() {
@@ -2441,7 +2453,7 @@ describe("A parser that does not throw on unsupported commands", function() {
     });
 });
 
-describe("The symbol table integraty", function() {
+describe("The symbol table integrity", function() {
     it("should treat certain symbols as synonyms", function() {
         expect(getBuilt("<")).toEqual(getBuilt("\\lt"));
         expect(getBuilt(">")).toEqual(getBuilt("\\gt"));
@@ -2475,10 +2487,30 @@ describe("A macro expander", function() {
         compareParseTree("\\foo", "x", {"\\foo": " x"});
     });
 
-    it("should consume spaces after macro", function() {
+    it("should consume spaces after control-word macro", function() {
         compareParseTree("\\text{\\foo }", "\\text{x}", {"\\foo": "x"});
     });
 
+    it("should consume spaces after macro with \\relax", function() {
+        compareParseTree("\\text{\\foo }", "\\text{}", {"\\foo": "\\relax"});
+    });
+
+    it("should consume spaces after \\relax", function() {
+        compareParseTree("\\text{\\relax }", "\\text{}");
+    });
+
+    it("should consume spaces after control-word function", function() {
+        compareParseTree("\\text{\\KaTeX }", "\\text{\\KaTeX}");
+    });
+
+    it("should preserve spaces after control-symbol macro", function() {
+        compareParseTree("\\text{\\% y}", "\\text{x y}", {"\\%": "x"});
+    });
+
+    it("should preserve spaces after control-symbol function", function() {
+        expect("\\text{\\' }").toParse();
+    });
+
     it("should consume spaces between arguments", function() {
         compareParseTree("\\text{\\foo 1 2}", "\\text{12end}", {"\\foo": "#1#2end"});
         compareParseTree("\\text{\\foo {1} {2}}", "\\text{12end}", {"\\foo": "#1#2end"});
@@ -2519,6 +2551,20 @@ describe("A macro expander", function() {
         });
     });
 
+    it("should allow for space second argument (text version)", function() {
+        compareParseTree("\\text{\\foo\\bar\\bar}", "\\text{( , )}", {
+            "\\foo": "(#1,#2)",
+            "\\bar": " ",
+        });
+    });
+
+    it("should allow for space second argument (math version)", function() {
+        compareParseTree("\\foo\\bar\\bar", "(,)", {
+            "\\foo": "(#1,#2)",
+            "\\bar": " ",
+        });
+    });
+
     it("should allow for empty macro argument", function() {
         compareParseTree("\\foo\\bar", "()", {
             "\\foo": "(#1)",
@@ -2526,6 +2572,16 @@ describe("A macro expander", function() {
         });
     });
 
+    // TODO: The following is not currently possible to get working, given that
+    // functions and macros are dealt with separately.
+/*
+    it("should allow for space function arguments", function() {
+        compareParseTree("\\frac\\bar\\bar", "\\frac{}{}", {
+            "\\bar": " ",
+        });
+    });
+*/
+
     it("should expand the \\overset macro as expected", function() {
         expect("\\overset?=").toParseLike("\\mathop{=}\\limits^{?}");
         expect("\\overset{x=y}{\\sqrt{ab}}")