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Fixes #17900; fixes #20195. The existing "can_fail" and "has_side_effects" primop attributes that previously governed this were used in inconsistent and confusingly-documented ways, especially with regard to raising exceptions. This patch replaces them with a single "effect" attribute, which has four possible values: NoEffect, CanFail, ThrowsException, and ReadWriteEffect. These are described in Note [Classifying primop effects]. A substantial amount of related documentation has been re-drafted for clarity and accuracy. In the process of making this attribute format change for literally every primop, several existing mis-classifications were detected and corrected. One of these mis-classifications was tagToEnum#, which is now considered CanFail; this particular fix is known to cause a regression in performance for derived Enum instances. (See #23782.) Fixing this is left as future work. New primop attributes "cheap" and "work_free" were also added, and...