Trace offset

[lks9]

• [x] A trace offset could be simply the number of trace elements passed.
• [x] In the CompactTrace, it makes sense to count the bytes plus IE count.

So the idea would be to add a TracePos class to cover both. Now, bytes plus element count is used for both.

• [x] Then we can add skip offset (--seek) and count (--count) options to print_trace.py and trace_replay.py, to replay only a part of the trace.
• [ ] The trace offset can also be added to the assertions_list to better locate the assertions.
• [ ] Also it would be nice to store subtraces without iterating over them using print_trace. Just a method subtrace.py with trace in, the start and end offset and subtrace out.

[lks9]

Ok, the rest is not going to get implemented. Reopen if needed.