struct op: serial -> seqnum (and drop use of term "serial" everywhere).
struct op: op -> type
struct op_desc: general replacement for file/op_num pair.
Change add_dependency to take two const op_desc *: means reshuffle since we can't adjust the args any more.

Add handling of reduced test cases (traverse lines trimmed), and add all the test cases I've been using.

This has proven to be intractible: various attempts to eliminate have failed, so detect at runtime and cease the traversal (and do the remaining ops outside a traverse).

Note: we can still deadlock on traversal vs transaction corner cases.
We handle transactions as single operation, which it logically is.

If we do the trace inside the lock, we show the actual sequence number when the transaction started.

Also changes first member of transaction to have valid start_group field,
and fix outdated comment.

(Invasive: means passing full op array all the way through).

Generalizes traverse in traverse fix from rusty@rustcorp.com.au-20090629073630-3eduhyypx2tp6u80

We now use the "group_len" field for both transactions and traversals.