Replace ParseNode<*> with a more accurate AnyParseNode and fix flow errors. (#1387)

* Replace ParseNode<*> with a more accurate AnyParseNode and fix flow errors.

* Allow "array" environment type spec to use any all symbol type.

Before this commit, it was constrained to use "mathord" and "textord", but a
recent change in HEAD resulted in a "rel" node being used in the spec for, e.g.
\begin{array}{|l||c:r::}\end{array}

* Address reviewer comments: rename `lastRow` to `row` in array.js.

src/ParseError.js

@@ -1,7 +1,8 @@
 // @flow
-import ParseNode from "./ParseNode";
 import {Token} from "./Token";
 
+import type {AnyParseNode} from "./ParseNode";
+
 /**
  * This is the ParseError class, which is the main error thrown by KaTeX
  * functions when something has gone wrong. This is used to distinguish internal
@@ -15,8 +16,8 @@ class ParseError {
         // Error position based on passed-in Token or ParseNode.
 
     constructor(
-        message: string,              // The error message
-        token?: Token | ParseNode<*>, // An object providing position information
+        message: string,               // The error message
+        token?: ?Token | AnyParseNode, // An object providing position information
     ) {
         let error = "KaTeX parse error: " + message;
         let start;