Performance improvements to Vector.

@koden-km Thought I'd open this to discuss the approaches that work, for example iterating over the `SplFixedArray` with a for loop and dereferencing using `[$index]` is faster than iteration with `foreach`, at least in the cases where iteration must terminate at `$this->size` rather than `capacity` - there are plenty of places where this idiom is used in `Vector` that would likely see improvement.

As a slightly longer-term goal I'd like to get the native PHP implementation of `Vector` as performant as it can be before having a serious go at a C implementation (be it using Zephir, or hand-rolled).