MAINT: Use intp output param viewable casts/methods (#20176)

This removes the cast-is-view flag with a more generic intp output.
Such an output could (theoretically) be nonzero, indicating e.g.
the offset of a complex number into a real one.
(We do not use this yet, though!)

The only "tricky" part is that the MinCastSafety helper used to
deal with the view-offset as well, and now we have to deal with it
explicitly when e.g. multiple fields have to be checked.

Bumps the experimental-dtype-api number, since the signatures changed.

* MAINT,DOC: Cleanups and clarfications based on Marten's comments

Co-Authored-By: Marten van Kerkwijk <mhvk@astro.utoronto.ca>

* MAINT: Rename `PyArray_GetCastSafety` to `PyArray_GetCastInfo`

Better captures the fact that it also returns the view-offset
information now.

* MAINT: Fix structured cast-is-view logic

* MAINT: Address review comments by Marten

Co-authored-by: Marten van Kerkwijk <mhvk@astro.utoronto.ca>

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