Add Vector.take and Vector.drop functions

[radeusgd]

Pull Request Description

Implements https://www.pivotaltracker.com/story/show/182307048

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[radeusgd]

I added this special case, because it can be triggered relatively often by vec.take (By_Index 123). It seemed that creating a slice just to hold a single element will be an overkill - because the cost of allocating the atom representing the slice descriptor should be roughly the same as the cost of allocating a single-element array and the memory footprint of both of these is also comparable, so it seems to me that making a slice for just a single element would be wasteful.

[jdunkerley]

We could do a faster short circuit earlier that if By_Index is an Integer or a single Integer Vector we return just self.at index and let the self.at do the checking and resolving,

[radeusgd]

We could, but then again I still need to do the check at slice_ranges for a single range - so I still need this case-of but now it will be in two places.

[jdunkerley]

Want to have a think on the normalize_ranges and invert but generally looks good.

[jdunkerley]

We could do a faster short circuit earlier that if By_Index is an Integer or a single Integer Vector we return just self.at index and let the self.at do the checking and resolving,