Reduce SVG (#819)

* Reduce SVG

1. Edit to reduce redundancy in SVG code. Save 5 KB in size of stretchy.js

2. Bug fix: Use correct path geometry for `\xleftharpoondown`.

* remove unnecessary fill='currentcolor'

static/katex.less

@@ -547,6 +547,14 @@
         display: block;
         position: absolute; // absolute relative to parent
         width: 100%;
+
+        path {
+            fill: currentColor;
+        }
+
+        line {
+            stroke: currentColor;
+        }
     }
 
     // Define CSS for image whose width will match its span width.