Automatic strides are broken if one of the strides is UNK

[dsharlet]

Unknown strides are reassigned in resolve_unknowns: https://github.com/dsharlet/array/blob/fba09f8051b4334c510c843d9befa8e4b5167941/array.h#L347-L348 which is incorrect if a stride is actually the value represented by UNK.

[Peter-McLean-Altera]

A candidate is std::dynamic_extent