cf git sha 55d3d77c470f416befd16a1b7d47f751eb2171fc of rdbg

More specifically, replace @ by rev_append and flatten+map by fold_left.

On a file that has 11000 equations (normal.ec), I win a factor 4: 60s -> 15s

The main difference is that I use a map instead of list to store
visited nodes.  The compilation time of normal.ec is divided by 2!!
(35s->15s)

We do not really see the difference in the time of the non-reg tests
since the execution of normal.ec still exceeds the 10s timeouts.

Fix a bug in socExec.ml (strangely untriggered before) along the way.