path: refactor for performance and consistency

Convergence PR prompted by nodejs/node#35 Original PR: joyent/node#9289

Original PR message (slightly modified):

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Improve performance by:

• Not leaking the arguments object in win32.join
• 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

Improve both by:

• Making the reusable trimArray() function
• Standardizing getting certain path statistics with the new win32StatPath() function

Benchmarks: (higher is better)

function (path.)	current (ops/sec)	this PR (ops/sec)
win32.resolve	315,019	399,543
win32.normalize	873,559	1,051,170
win32.isAbsolute	1,915,332	1,939,849
win32.join	452,879	815,033
win32.relative	153,821	219,928
win32.format	13,118,588	16,364,384
posix.relative	244,638	304,456

Benchmark code (gist)