Build the webmain executable. The webserver can be started with:

  ./_build/default/webmain.exe --server

and it should be accessible at http://127.0.0.1:8080

The old `Makefile`-based build system fails to build with newer
OCaml and Menhir versions. Add support for building with the new `dune`
build system. For now, only the main executable is built. As a future
improvement, we need to support the build of the HTTP server and the
tests.

Fix also two build errors.

Import the tarball of banal-1.0 from https://www-apr.lip6.fr/~mine/banal/