Copy byte buffer

memcpy( dest[], source[], index, length )

Copy the source string to dest, starting at the index byte in dest, and counting length bytes.

Returns number of bytes copied.

    new s1{20};
    new s2{20};
 
    strcopy(s1, "ABCdef");
    memcpy(s2, s1, 3, 3);    
 
    // s2 now contains "def" - NOTE that no terminating NULL character is copied