Implement int64 indexing

[lstrgar]

int32 indexing over fields limits the size they can take. I regularly run into this limitation.

SNode index might be out of int32 boundary but int64 indexing is not supported yet. Struct fors might not work either.
...
[E 12/18/24 23:27:59.749 2523792] [demote_dense_struct_fors.cpp:convert_to_range_for@29] Assertion failure: total_n <= std::numeric_limits<int>::max()

See related issues:

https://github.com/taichi-dev/taichi/issues/5320 https://github.com/taichi-dev/taichi/issues/6258

[bgailleton]

Hi all, any news on that side?

[lstrgar]

Yes I would like to bump this. Is this an easy fix? @FantasyVR