Audit uses of 32-bit indexing

[maleadt]

We're currently using Int32 indices in some kernels, using the i32 hack, because that often results in significantly better performance. However, GPUs are getting large, and users are starting to use arrays that overflow typemax(Int32) elements. This can results in bugs like #1963

We should be more careful about using 32-bit indexing, and probably not use i32 until we have a better way of deciding which index type to use. Maybe we can add some kind of index_type trait, defaulting to Int but possibly using Int32 when the input arrays allow it, e.g., using #1895.