Delete slots in the P/Invoke tool

My P/Invoke command-line tool previously had no way to remove or redefine a slot once it was created. There currently isn't much use for that since scripts proceed only directly forward, but eventually I'd like it to be possible to write loops and branches. If there's looping going on, it might be necessary to reallocate a buffer. Also, in interactive mode, it would be nice to be able to redefine a slot with a different type if a mistake was made in creating it. So, I recently added the deleteslot instruction, which removes a given slot from the current workspace and frees the shred of memory used to store its data.