Write to slot offsets in the P/Invoke tool

My P/Invoke command line tool's copyslot instruction already had a way to take one field or offset from a source slot, but there was no way to set just one field of a block. Today I added symmetry to that instruction by allowing the destination slot to also accept field or offset keywords. One field of one block can be copied to a field in a different block, or a non-block slot's value can be set into a field of a destination block.