As_path overhaul.

The new scheme is going to be less error prone (no exceptions) and
more convenient if you have to work for a while in a single space
(e.g. relative). Includes many other changes. One thing that
still needs to be done is .. and . handling.
Revert the metadata dep mecanism introduced in 9a594c2.

In fact it is not needed if the values in the metadata are
used this will be naturally propagated, otherwise there's no
reason they should be part of the configuration.
Tentatively add something that allows to install "static" files.

Conceptually not entirely satisfactory but works with `Dir. We need to
say something about noop actions.
Tweak Action.cmd defs a bit. Add an echo configuration key.
Less idiotic ANSI escapes/Format integration.
Part's actions is now a configuration value.

And many other changes.
Replace ~cond arguments by ~exists.
Basic actions for units and libs.
Refine merlin generation.

Now that we know how to distinguish source from build products.
Remove Conf.{List,Option}.

They seem less useful now that most work happens in the appfun.
Rename As_action.link to As_action.symlink.

Less confusing in a compiler setting.
Add a proof of concept `product` command to the driver.
Redo dir part again.

This time we have a correct semantics w.r.t. to static files and
the build system. Still need get a few good combinators for dir
specifiers though.
Hack an evaluation cache.
Add As_action_ocaml.link_{byte,native}.
Basic actions for OCaml bin.
Add support in `Unit and `Bin for seeing project libraries.
Outline a strategy for packages.
OCaml find lookup.

Trying a new query strategy.
Do not forget to lookup cache value on Conf.app.

Where you need it most.