module: avoid JSON.stringify() for cache key

By avoiding JSON.stringify() and simply joining the strings with a
delimiter that does not appear in paths, we can improve cached
require() performance by at least 50%.

Additionally, this commit removes the last source of permanent
function deoptimization (const) for _findPath().

PR-URL: https://github.com/nodejs/node/pull/10789
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/module.js

@@ -165,10 +165,11 @@ Module._findPath = function(request, paths, isMain) {
     return false;
   }
 
-  const cacheKey = JSON.stringify({request: request, paths: paths});
-  if (Module._pathCache[cacheKey]) {
-    return Module._pathCache[cacheKey];
-  }
+  var cacheKey = request + '\x00' +
+                (paths.length === 1 ? paths[0] : paths.join('\x00'));
+  var entry = Module._pathCache[cacheKey];
+  if (entry)
+    return entry;
 
   var exts;
   var trailingSlash = request.length > 0 &&