Speed up Array.prototype.slice on ArrayBuffers

[gfwilliams]

It looks like .slice iterates to get to the start index (because it's used to working with Sparse arrays).

jsvIterator should provide a jsvIteratorGotoIndex function that goes to the index in the fastest way for what it's iterating over. It could speed up quite a lot of built in functionality when it's used on Typed Arrays.

... also it'd make #58 work much better as well.