Adding a slice equivalent for `Vector`.

[AtheMathmo]

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It would be good to add a slicing equivalent for Vector (similar to MatrixSlice for Matrix).

For Vector we can probably do this in a nicer way than Matrix. We can implement a fat pointer type to represent the underlying data in a Vector - and then implement the relevant methods on a reference to this type (similar to the Vec - &[T] relationship in std).

Using Deref/Borrow/AsRef we can cut down on code by implementing all functions taking &self on the VectorSlice type.

It would be acceptable to simply mirror the current behaviour for matrices - but we should try to get the above working.