While diagnosing some (unrelated) memory usage in htools, I've
stumbled upon some very bad behaviour in checkData: mapAccum is
non-strict, and the tuple we use also, so that results in the list of
list of messages being very bad space-wise (hundreds of MB of memory
for a simulated cluster with thousands of nodes, all with errors).

The new, explicit reuse of the old message list has a linear memory
behaviour. The only downside is that messages are listed in the
reverse order (which I'll fix on master).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>