Improve allocation speed and reduce memory copying to increase HTTP pars...

...er performance by 1-2 (and occasionally more) percent in many cases such as benchmark/http/end-vs-write-end.js based on slab allocator todo by Ben.
Ideally, the benchmark should work on all platforms.  If you grep test/common.js for `PIPE`, you can find paths that work on Windows and Unices:

```
$ git grep -nw PIPE test/common.js | head -2
test/common.js:44:  exports.PIPE = '\\\\.\\pipe\\libuv-test';
test/common.js:47:  exports.PIPE = exports.tmpDir + '/test.sock';
```