path: refactor for performance and consistency

Improve performance by:
+ Not leaking the `arguments` object!
+ Getting the last character of a string by index, instead of
  with `.substr()` or `.slice()`

Improve code consistency by:
+ Using `[]` instead of `.charAt()` where possible
+ Using a function declaration instead of a var declaration
+ Using `.slice()` with clearer arguments
+ Checking if `dir` is truthy in `win32.format`
  (added tests for this)

Improve both by:
+ Making the reusable `trimArray()` function
+ Standardizing getting certain path statistics with
  the new `win32StatPath()` function

PR-URL: https://github.com/nodejs/io.js/pull/1778


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>