fix: avoid repeated String#slice calls in stringDistance. (#1095)

The `stringDistance` function calls

```js
strA.slice(0, -1)
```

and

```js
strB.slice(0, -1)
```

multiple times, which is a bit of a waste of time here. JavaScript
engines cannot generally eliminate the duplicated calls easily, so
it's better to avoid the redundant calls altogether.

This improves the chai test on the
[web-tooling-benchmark](https://github.com/v8/web-tooling-benchmark) by
around 8% when run with upcoming V8 6.4.

lib/chai/utils/proxify.js

@@ -112,10 +112,12 @@ function stringDistance(strA, strB, memo) {
     if (strA.length === 0 || strB.length === 0) {
       memo[strA.length][strB.length] = Math.max(strA.length, strB.length);
     } else {
+      var sliceA = strA.slice(0, -1);
+      var sliceB = strB.slice(0, -1);
       memo[strA.length][strB.length] = Math.min(
-        stringDistance(strA.slice(0, -1), strB, memo) + 1,
-        stringDistance(strA, strB.slice(0, -1), memo) + 1,
-        stringDistance(strA.slice(0, -1), strB.slice(0, -1), memo) +
+        stringDistance(sliceA, strB, memo) + 1,
+        stringDistance(strA, sliceB, memo) + 1,
+        stringDistance(sliceA, sliceB, memo) +
           (strA.slice(-1) === strB.slice(-1) ? 0 : 1)
       );
     }