build: add small clean-up for `doc`
test: add a “prepare backup on some host” workflow
build: add the `sig` subcommand

The command calls `ocamlc -i` with the right arguments to see the
inferred signature of an ML file.
lib: add URI parsing `Host`

This also renames `to_string` to `to_string_hum`.
lib: add (back) `?id` parameter to `Target.create`
lib: create module `Ketrew.EDSL`
test: use EDSL in the `make_tar_gz` workflow
lib: kill targets whose dependencies are missing

If the user uses the EDSL the case could not happen, but using the
lower-level library, it was possible to put the engine in an annoying
state.
test: add a case of dependency missing in the DB
lib: improve `EDSL.host_cmdliner_term`