module: optimize js and json file i/o

Use internalModuleReadFile() to read the file from disk to avoid the
fs.fstatSync() call that fs.readFileSync() makes.

It does so to know the file size in advance so it doesn't have to
allocate O(n) buffers when reading the file from disk.

internalModuleReadFile() is plenty efficient though, even more so
because we want a string and not a buffer.  This way we also don't
allocate a buffer that immediately gets thrown away again.

This commit reduces the number of fstat() system calls in a benchmark
application[0] from 549 to 29, all made by the application itself.

[0] https://github.com/strongloop/loopback-sample-app

PR-URL: nodejs#4575
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/module.js

@@ -428,14 +428,14 @@ Module.prototype._compile = function(content, filename) {
 
 // Native extension for .js
 Module._extensions['.js'] = function(module, filename) {
-  var content = fs.readFileSync(filename, 'utf8');
+  var content = internalModuleReadFile(filename);
   module._compile(internalModule.stripBOM(content), filename);
 };
 
 
 // Native extension for .json
 Module._extensions['.json'] = function(module, filename) {
-  var content = fs.readFileSync(filename, 'utf8');
+  var content = internalModuleReadFile(filename);
   try {
     module.exports = JSON.parse(internalModule.stripBOM(content));
   } catch (err) {