Implement `fold` and `reduce` methods for tables

[pathscale]

Right now it's impossible to iterate over the records and collect some data out of them as iter_with takes Fn as a function, so no mutations. Replacing Fn with FnMut might not be the best solution as in some cases the function shall not mutate anything. That's why we should consider providing fold and reduce methods.

As for now it's possible to workaround the limitation via Cell.

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Migrated from pathscale/wt-dead#118. Originally opened by @YohDeadfall on 2025-11-03T22:09:22Z.

[pathscale]

Confirmed still open with a compile probe on the current PR #46 merge ref. Generated tables expose iter_with and iter_with_async, but calls to table.fold(...) and table.reduce(...) both fail with E0599 because no such table methods exist.