Fix unsafe blocks in s![] macro

[jturner314]

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Before, the user could silently violate safety requirements of SliceInfo::new_unchecked by directly calling s![@parse inconsistent_values], where inconsistent_values represents inconsistent values for the dimensions, etc. Now, the macro calls SliceInfo::new_unchecked only in the ($($t:tt)*) arm, which always constructs the correct values for the call.

[jturner314]

CI results:

• The clippy warning is a bug (bad diagnostic) in clippy on the current Rust beta; it's fixed in the latest nightly.
• The cross_test error is something wrong with the GitHub action -- it's not an issue with ndarray.