[Type] Remove VecView class and update codebase to use Vec utilities instead

[alxbilger]

Benchmarks show that VecView is slower than using a Vec copy. However, VecView had the advantage to be more elegant and concise. To remove VecView, this PR introduces setsub in Vec. Note that this is consistent with the getsub method, and that Mat has also getsub and setsub.

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