💡`indexed_handler`: Coalesce two or more callbacks with equivalent indexed field conditions into one callback

[lukevalenty]

I have a few use cases where the indexed fields for two or more callbacks will have equivalent conditions on those fields. Instead of allocating 2 or more entries in the callback array, we can combine the callbacks into a single callback with a single entry in the array. This can also give the compiler some additional opportunities to optimize the resulting assembly.