pio_encode_delay() function

the delay slot bits to be ORed with an instruction encoding

This function does not return a valid instruction encoding; instead it returns an encoding of the delay slot suitable for `OR`ing with the result of an encoding function for an actual instruction. Care should be taken when combining the results of this function with the results of pio_encode_sideset and pio_encode_sideset_opt as they share the same bits within the instruction encoding.