Use Dune to generate OPAM package definitions

[dhil]

This patch moves the definitions of our OPAM packages into the file dune-project. The OPAM package definitions are automatically generated from this file. This approach offers some advantages:

• Less redundancy: package descriptions are now shared.
• Dune integration: OPAM package definitions are kept in sync with dune.
• Versioning: dune-release automatically versions the OPAM package definitions.

Our build rules for the links package are a little more complicated than dune supports out-of-the-box. The steps required to build this package are defined in the links.opam.template file, which dune automatically pastes into the generated links.opam file.

[dhil]

I fixed the minimum required version of OCaml to 4.14.0, as that seems to be the minimum version with which Links compiles at the moment.