Eval is less evil than monkey-patching a global

Closes isaacs#27

Due to the way that streams and the various FS methods interact with one
another, NOT monkey-patching doesn't work.

However, since the 'fs' module is effectively a process-level global,
making monkey-patchy changes to it can introduce some really weird behavior.
Even if such behavior is not actively harmful, it can end up being
applied multiple times, and wrapping 'fs.open()' over and over again
is a bit of a bummer when it comes to performance.

Mixing 'eval()' and a private/internal Node API kind of gives me the
heeby jeebies.  If anyone can come up with a less evil way to do this,
I'll be happy to accept a patch.

Author: isaacs

fs.js

@@ -0,0 +1,5 @@
+// eeeeeevvvvviiiiiiillllll
+// more evil than monkey-patching the native builtin?
+// Not sure.
+
+eval(process.binding('natives').fs)

graceful-fs.js

@@ -1,14 +1,12 @@
 // Monkey-patching the fs module.
 // It's ugly, but there is simply no other way to do this.
-var fs = module.exports = require('fs')
+var fs = module.exports = require('./fs.js')
 
 var assert = require('assert')
 
 // fix up some busted stuff, mostly on windows and old nodes
 require('./polyfills.js')
 
-// The EMFILE enqueuing stuff
-
 var util = require('util')
 
 function noop () {}

polyfills.js

@@ -1,4 +1,4 @@
-var fs = require('fs')
+var fs = require('./fs.js')
 var constants = require('constants')
 
 var origCwd = process.cwd

test/open.js

@@ -2,7 +2,7 @@ var test = require('tap').test
 var fs = require('../graceful-fs.js')
 
 test('graceful fs is monkeypatched fs', function (t) {
-  t.equal(fs, require('fs'))
+  t.equal(fs, require('../fs.js'))
   t.end()
 })
 

test/readdir-sort.js

@@ -1,5 +1,5 @@
 var test = require("tap").test
-var fs = require("fs")
+var fs = require("../fs.js")
 
 var readdir = fs.readdir
 fs.readdir = function(path, cb) {