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The new module Tool_cmdline replaces former Tool_common, Tool_main and
all uses of Cmdliner in Tool_*.

Small changes in output: lines are now prefixed by "I", "W" or "E".

 * big refactoring of Tool_credgen
 * --count now generates numeric ids: $T.hashcreds is always
   generated, $T.privcreds has always the same format

Both variants are declared as internal libs, and we now just have to
tag each executable with the desired version.

Add module Platform: single interface to functions that depend on the
runtime. For now, the native (existing one), and a dummy one that
constitutes a skeleton for the js one.

All calls to Cryptokit, Z, Calendar (which depends on Unix) in the
command-line tool now go through this module.

Side-effects:
 - more uniform handling of --help and --version, automatic generation
   of Unix manpages
 - slight change in syntax (see demo.sh)
 - sub-commands of "election" (Tool_election) are now direct commands,
   "election" itself disappears
 - in Tool_election, concatenate with --dir option (if any) instead of
   calling chdir

It matters behind a reverse-proxy.

Web_election.Make and SITE.import_election now return something that
can be discarded without side-effects on the running server.

Rationale: we want to be able to perform additional checks in the
caller of SITE.import_election. This will be useful when online
creation of elections is possible.

As a side-effect, they do no longer need to fit into memory. This
fixes "TODO: streaming".