New primitive - copy a register.

So now we can do "set reg1 = reg2", via "store #1, #2" and it works
as the following program demonstrates:

        # reg1 = steve...
        store #1, "steve\n"
        print_str #1

        # now reg2 == reg1
        store #2, #1
        print_str #2

        # and so a comparison is OK
        cmp #1, #2
        jmpz OK

        store #1, "ERROR reg1 != reg2"
        print_str #1
        exit

     :OK
        store #1, "'store #1, #2' worked!\n"
        print_str #1
        exit

This means we now have:

    store #1, "String"   # string
    store #1, 0x3342     # number
    store #1, label      # label
    store #1, #2         # register