Fixed lifetime unsoundness in arr macro.

[rodrimati1992]

Fixed the lifetime unsoundness demonstrated in the arr! unsoundness issue

Added doctests (that don't appear in the public documentation) for cases that are expected to compile,and not compile.

The doctests could probably be replaced with something less hacky(like trybuild),but I'm not going to do that.

[greyblake]

@fizyk20 @rodrimati1992 Would you consider it possible to backport the fix to 0.9 - 0.13 versions?

This will be very appreciated, because a big chunk of the ecosystem in direct or indirect way still relies on the old versions.
For example as-slice.

Thanks in advance.

[fizyk20]

I had the idea to do so yesterday evening and I'm working on it now :) I think that will be the fastest course of action here.

[greyblake]

@fizyk20 Thank you! Very appreciated!