Add m::raw_allocation tests, fix unique_span constructor interface when end iterator is a sentinel type

[EmJayGee]

Realized that the constructor for unique_span<> that takes two iterators did not correctly support when end is a different type than the beginning point (presumably because it is a sentinel type).

Also added more unit tests for both unique_span and raw_allocator, which found some interface problems with how the value type was handled inside the class. That seems to be worked out now.

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This discussion was created from the release Add m::raw_allocation tests, fix unique_span constructor interface when end iterator is a sentinel type.