Add .reborrow() methods to array views

[bluss]

ArrayView<'a, T, D> is invariant in parameters T and 'a, because
ArrayBase<S, D> is invariant in the parameter S because the array
base is using the projection <S as Data>::Elem in the struct
definition.

This means that the equivalent of the &'a T -> &'b T conversion (valid
whenever 'a: 'b is not naturally available for array views; we can
provide it manually, with a method.

[bluss]

cc @jturner314; I'm unsure here if "reborrow" is the right name.

[jturner314]

"reborrow" seems like a good name to me. You are effectively borrowing the data again; the 'a: 'b constraint is just necessary to ensure that the lifetime of the new borrow is valid. You could call it "shrink_lifetime", "reduce_lifetime", or "shorten_life", but I like "reborrow" better.

For those not on IRC, this was the case that caused me to encounter the issue. I did find that calling .view() on state (Foo { mean: state.view(), covariance: view }) was a workaround for this particular case, but I'm not sure that workaround would work for all cases.

[Robbepop]

I was really in need of this API some time ago. Thanks for introducing it! :)

[bluss]

Good. It is a temporary solution.