Workflow: interface improvement and documentation (especially shell script combinators)
backup commit, just in case (R). Does not compile
db: rewrite interface and documentation
db: enriched logging messages
workflow: possible interpreters are typed
harmonized engines signatures, notably wrt logging
engine_lwt: rewritten to resemble the sequential version
engine: fixed missing case when a workflow fails to produce target at prescribed location
log: a build failure entry can now have a comment