memcpy primitive added.  use peek/poke consistently.

The following code now compiles and executes:

        goto run
    :code
        store #1, "Steve Kemp\n"
        print_str #1
        exit
    :run
        store #1, 0x5000
        store #2, code
        # len is guess here
        store #3, 50
        memcpy #1, #2, #3

        goto 0x5000

This closes #10.