This patch is a proof-of-concept of another way to
remove the SET_LINENO patch (as opposed to Vladimir's
ancient one).

Instead of rewriting bytecode (ick!) we poke into the
c_lnotab to see if we've moved onto a different line.

The c_lnotab is not the most transparent of data
structures, it has to be said.

I'm not sure this patch is 100% correct -- but I think
the idea can definitely fly.  There will be some more
overhead to tracing than before, but I hope not too
much.  I haven't tested these aspects.

Comments welcome!