use Exn.interruptible_capture to keep user-code interruptible (Exn.capture not immediately followed by Exn.release here);

added Exn.interruptible_capture, which reraises interrupts as required by user-code (when Exn.capture is not immediately followed by Exn.release); refined Exn.is_interrupt: detect nested IO Interrupts; generalized Exn.map_result; more precise dependencies;

eliminated dead code;