Check that built-in callable types validate their output type is `Sized` (in new solver)
Working on parity with old solver. Putting this up for consideration, it's not *really* needed or anything just yet. Maybe it's better to approach this from another direction (like always checking the item bounds when calling `consider_assumption`? we may need that for coinduction to be sound though?)
This basically implements #100096 for the new solver.
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